As a picture speak a thousand words, let's we see the following pictures and try to understand what they described.

Human sexuality is how people experience and express themselves as sexual beings. Human sexuality has many aspects. Biologically, sexuality refers to the reproductive mechanism as well as the basic biological drive that exists in all species and can encompass sexual intercourse and sexual contact in all its forms. There are also emotional or physical aspect of sexuality, which refers to the bond that exists between individuals, which may be expressed through profound feelings or emotions, and which may be manifested in physical ormedical concerns about the physiological or even psychological aspects of sexual behaviour.Sociologically, it can cover the cultural, political, and legal aspects; and philosophically, it can span the moral, ethical, theological, spiritual or religious aspects.

Recent studies on human sexuality have highlighted that sexual aspects are of major importance in building up personal identity and to social evolution of individuals:

“Human sexuality is not simply imposed by instinct or stereotypical conducts, as it happens in animals, but it is influenced both by superior mental activity and by social, cultural, educational and normative characteristics of those places where the subjects grow up and their personality develops. Consequently, the analysis of sexual sphere must be based on the convergence of several lines of development such as affectivity, emotions and relations”.

The biology of human sexuality examines the influence of biological factors, such as organic and neurological response, heredity, hormones, and sexual dysfunction; it examines the basic functions of reproduction and the physical means to carry it out. The biological perspective helps to analyze the factors, and ultimately aids in understanding them and using them to deal with sexual problems.

Sex as exercise burns calories to produce health benefits. Sex also relieves stress, boosts theimmune system with higher levels of immunoglobulin A, improves cardiovascular health, increasesself-esteem, improves intimacy, reduces pain by production of the hormone oxytocin, reduces the risk of prostate cancer, strengthens pelvic muscles, and promotes good sleep. Sex also improves the sense of smell and urinary bladder control.

Sexual behavior can be a disease vector. Safe sex is a relevant harm reduction philosophy.

Foreplay

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In human sexual behavior, foreplay is a set of intimate psychological and physical acts between two or more  

people meant to increase sexual arousal.

 

Foreplay involves different acts such as kissing, touching,embracing, talking, and teasing (teasing, in this case, may include methods of satisfaction, such as erotic sexual denial). Sexual stimulation of all kinds, such as manual or oral stimulation of erogenous zones are considered foreplay. Sexual role playing, fetish activities, and BDSM can also be considered foreplay, though they may also accompany intercourse and not just precede it. Of the various forms of foreplay, the most common include fellatio and cunnilingus.

Progression

Foreplay eventually turns physical. Simple and possibly innocuous acts, such as straightening someone's clothing or hair, bumping into someone while walking, stroking someone's arm, or whispering in someone's ear can constitute foreplay. One may also hold hands, touch the face,kiss, "bite", tickle, or massage.

As comfort increases, so usually does the level of intimacy. More intimate examples include:
 
 

• Deep tongue kissing, also known as French kissing;
• Touching and massaging erogenous zones over clothing, also known colloquially as groping or"heavy petting";
• Rubbing together erogenous zones over clothing, also known as dry humping or grinding;
• Undressing oneself or partner, also known as stripping.
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There are many types of foreplay. Stimulation with the hands is the most common form, followed by oral stimulation. Foreplay is to provide sexual pleasure and arousal. The act is generally the act of preparing one's partner for sexual intercourse, though it does not exclude the chance of orgasm if applied rigorously.
 

Direct manipulation of naked erogenous zones is almost always considered foreplay (except in a medical context). In women, this includes stimulation of the clitoris and vulva. In men, it includes stimulation of the penis and scrotum. For both sexes, it could include stimulation of nipples andanus. Stimulation can be achieved by mouth, hands, sex toys like dildos or vibrators, or common household objects like feathers or ice cubes. Direct manipulation of naked erogenous zones is not considered foreplay when it is not preparatory for further sexual acts. For example, mutual masturbation and oral sex are often considered final sexual acts; as final acts with no expectation of further sexual congress, these are not considered foreplay.

Foreplay tends to become increasingly physical and intense as the couple moves closer to intercourse. It reaches its peak in the moments just before intercourse, when it induces a strong mutual desire for penetration. During the final stage of foreplay, genital teasing may take place for a brief time. Technically, foreplay ends with intromission, or the beginning of intercourse. In practical terms, however, the continuity between foreplay and intercourse may be very great, since the couple may continue to engage in foreplay-like behavior during intercourse.

Safe sex practices can be incorporated as part of foreplay. A condom or dental dam can be applied in an erotic or playful way as part of the final stages of foreplay.

Function and effects

Psychologically, foreplay lowers inhibitions and increases the emotional comfort of the partners. Physically and takes close to intercourse, it stimulates the process that produces an erection in men, allowing them to penetrate an orifice. In women, it helps stimulate the process that leads to erection of the clitoris, raising of the cervix (elongation of the vaginal canal), and the production of vaginal lubrication, allowing penetration to take place comfortably.

 
Erotic sexual denial

Erotic sexual denial, also known as orgasm denial, is a sexual practice in which a person is kept in the plateau phase of sexual arousal for an extended length of time without satisfying climax. This is sometimes practiced in association with BDSM, and can be thought of as a more extreme version of orgasm control, which ends in satisfying climax. The subject may either be allowed an orgasm at the end (in which case, the orgasm is generally much stronger than normal), or denied one, in which case they will generally feel strong feelings of sexual frustration.

An alternative form of erotic denial is the total denial of all genital stimulation. To ensure a total denial of stimulation a couple may use a chastity device that physically prevents touch and/or full erection.

Erogenous zone

An Erogenous zone is an area of the human body that has heightened sensitivity and stimulation of which normally results in sexual response. There is individual variation in sensitive areas, but the majority of men and women have common erogenous zones, both areas of the skin, and the penis in men and clitoris in women.

There are two types of erogenous zone response in the skin: nonspecific and specific.

Classification

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Nonspecific

The skin is similar to normal haired skin and has the normal high density of nerves and hair follicles. These areas include the sides and back of the neck, the axillae (armpits) and sides of the thorax. An exaggerated tickle and anticipatory response are responsible for the heightened sensual response.

Specific

These areas produce stronger sensation and include the genitals, including prepuce, penis, clitoris,vulva and perianal skin, scrotum, lips, and nipple. The rete ridges of the epithelium are well formed and more of the nerves are close to the external surface of the skin than in normal haired skin.

Private Area

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Female genitalia

The clitoris, is a visible button-like structure located above the labia, and is covered by a small fold of skin known as the clitoral hood. It has the densest nerve supply of any part of the skin.

Located past the clitoris, just above and on either side of the urethral opening is sensitive erectile tissue known as the Skene's gland, or U-Spot.

Male genitalia

The penis is the most sensitive erogenous zone in the male body. In particular, the natural glans(head) and frenulum (foreskin) are highly sensitive and may elicit strong sensations from the slightest touch or movement. The ridged band, theorized by John R. Taylor, is believed to be a sensitive part of the penis as well.

Anus

Moving from the hairy skin to the glabrous skin around the anus the nerve networks rise higher in the skin and the mucocutaneous end-organ becomes apparent at the vermilion border, occurring frequently in this transition zone. The Vater-Pacini corpuscle is deep in the subcutaneous tissues, and into the anal canal.

Waist

Nibbling and biting with moderate pressure(love bites)so that the partner does not feel pain is very erotic on the sides of the waist. Licking and tracing fingers over a partner's body is also very stimulating.

Lips

Biting the lower lip softly.

Mouth

Sticking your tongue in your partner's mouth and licking the top of mouth.

Neck

The neck and clavicle area is a very sensitive region. Licking, kissing, and caressing here are very arousing to many people, hence the term "necking". The neck area is very prone to broken blood vessels; rough kissing can leave bruises known as love bites, commonly referred to as "hickies" or "monkey bites" . Additionally the back of the neck is very sensitive to arousal by lightly caressing.

Ears

Licking and sucking gently on earlobes is arousing to many people. Also, whispering or breathing softly in one's ear can also be very pleasurable.

 

Chest

The areola and nipple contain Golgi-Mazzoni, Vater-Pacini and genital corpuscles. No Meissner's corpuscles and few organized nerve endings are present. There are concentrations of nerve tissue in the area of ducts and masses of smooth muscle. The hair surrounding the areola adds additional sensory tissue. The mass of smooth muscle and glandular-duct tissue in the nipple and areola block the development of normal dermal nerve networks which are present in other erogenous regions and the development of special end organs. Indeed, the entire breast has a network of nerve endings, and they have the same number of nerve endings no matter how large they are. Accordingly, smaller breasts are more sensitive while larger breasts can be stimulated (e.g. fondled, bitten, sucked) more forcefully. Intense stimulation of a woman's nipples may result in a surge in the production of oxytocin and prolactin which could have a significant effect on her genitals. Placing a man's face inbetween the cleavage also incrases sexual pleasure and breast size.

Armpits

Armpits have an errogenous effect by releasing a male pheromone. When absorbed nasally by a female who is ovulating, this will have sexually stimulating effect. On the contrary, a female not ovulating will typically find this unpleasant

 

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Fingers

The fingertips have many nerves and are responsive to very light touches, like the brushing of a tongue or light fingernail or teeth stroking. The sides are somewhat less sensitive and more ticklish. Both light and firmer touches work well at the junction of the fingers.

Feet & Toes

Due to the concentration of nerve endings in the sole and digits of the human foot, the sensations produced by sucking or licking the feet can be pleasurable. Similarly, massaging the sole of the foot produces similar nervous stimulation.

Sexual arousal

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Sexual arousal is the the arousal of sexual desires in preparation for sexual behavior.

Sexual arousal for a man is usually associated with a penile erection. This is usually the most prominent and reliable sign of sexual arousal in males; however, adolescent males experience frequent 'non-sexual' erections stemming from their high level of testosterone. In a woman, the vagina becomes lubricated in anticipation of sexual intercourse. Unlike most animals, human beings of both sexes are potentially capable of sexual arousal throughout the year; therefore, there is no human mating season. Things that precipitate human sexual arousal are colloquially known as turn-ons. Turn-ons may be physical or mental in nature. For example, psychologically, most heterosexual males are visually stimulated by female nudity or partial nudity, while some heterosexual females are stimulated by the state of romance with their partner. According to the marital and sex therapist, David Schnarch, intimacy, the honest portrayal of the two people joining in the sexual act, may lead to a heightened state of passion in sex, including sexual arousal. Given the right stimulation, sexual arousal in humans will typically end in an orgasm, but may be pursued for its own sake, even in the absence of an orgasm.

Sexual arousal causes different physical changes.

 
 

 

 

In females:	In males:
Increase in breast size Erection of nipples Vaginal lubrication Vasocongestion of the vaginal walls Tumescence and erection of the clitoris andlabia Elevation of the cervix and uterus, and expansion of the back of the vagina Change in shape, color and size of the labia majora and labia minora	Penile tumescence and  erection The veins in the penis may become more prominent Retraction and tightening of the foreskin, often exposing the glans penis if not normally exposed (unless the particular male is circumcised) Emission of pre-ejaculatory fluid Swelling of the testes Ascension of the testes Tensing and thickening of the scrotum NEXT Erection and more