General
Fellatio can be sexually arousing for both participants, and males who receive fellatio commonly experience orgasm and ejaculation of semen during the act. People may use fellatio as foreplay to sexually arouse their sex partner before vaginal or anal intercourse, or other sexual activity, or they may perform it as an erotic and physically intimate act in its own right. The sex partner may be of either sex. When the penis is thrust into someone's mouth, it may be called irrumatio, though the term is rarely used.
The essential aspect of fellatio is oral stimulation of the penis through some combination of sucking, use of the tongue and lips, and movement of the mouth. One method is the sex partner taking the penis into the mouth and moving smoothly up and down the penis to a rhythm while being careful to avoid contact with the teeth. Fellatio may also include the oral stimulation of the scrotum, whether licking, sucking or taking the entire scrotum into the mouth. During the act, semen may be ejaculated into the partner's mouth.
It is difficult for some people to perform fellatio due to their sensitivities to the natural gag reflex. Different people have different sensitivities to the reflex, but some people learn to suppress the reflex. How much of the penis a partner can take into their mouth also depends on the size and length of the penis, and they most commonly only take the end of the penis into their mouth.
It is physically possible for men with sufficient flexibility, penis size, or both, to perform fellatio on themselves as a form of masturbation, in an act called autofellatio. However, few men possess sufficient flexibility and penis length to safely perform the necessary frontbend.
Deposition of semen
During fellatio, a partner may ingest semen from the penis. Nancy Friday's book, Men in Love – Men's Sexual Fantasies: The Triumph of Love over Rage (1982), suggests that swallowing semen is high on a man's intimacy scale. As late as 1976, some doctors were advising women in the eighth and ninth months of pregnancy not to swallow semen lest it induce premature labor, but it was later determined to be safe.
The male whose penis is being stimulated during fellatio typically becomes sexually aroused. Once the prerequisite level of sexual stimulation has been achieved and ejaculation becomes imminent, the semen may be discharged onto his partner. The male may position his penis prior to ejaculation so that semen will be deposited onto his partner's face (known as a "facial"), or other body part such as their neck, chest or breast.
Deep-throating
Deep-throating is a sexual act in which a person takes a partner's entire erect penis into the mouth and throat. The technique and term became popularized by the 1972 pornographic film Deep Throat. Generally, the person receiving fellatio is in control. For deep-throating, the penis must be long enough so that it can reach the back of the giver's throat.
Deep-throating can be difficult, due to the natural gag reflex triggered when the soft palate is touched. People have different sensitivities to this reflex, and with practice, some learn to suppress it. Deep-throating leads to an entirely different kind of oral stimulation in comparison to regular fellatio: the tongue is immobilized during deep-throating and sucking becomes impossible; the glans penis can be intensely stimulated by the tightness of the pharynx.
Other aspects
An illustration of two men performing fellatio on each other
The man receiving fellatio receives direct sexual stimulation, while his partner may derive satisfaction from giving him pleasure. Giving and receiving fellatio may happen simultaneously in sex positions like 69 and daisy chain.
Fellatio is sometimes practiced when vaginal or anal penetration would create a physical difficulty for a sex partner. For example, it may be practiced during pregnancy instead of vaginal intercourse by couples wishing to engage in intimate sexual activity while avoiding the difficulty of vaginal intercourse during later stages of pregnancy.
Other reasons why a woman may not wish to have vaginal intercourse include apprehension of losing her virginity or of becoming pregnant, or because she may be menstruating.