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Oral sex has been considered to be taboo to a greater or lesser extent in many Western countries since the beginning of the Middle Ages. In the West before that time, and within certain constraints, the act of oral sex was a more or less widely accepted activity in those cultures that practiced regular and consistent bathing.
Even then, there were still notable taboos: in pre-Christian ancient Rome sexual acts were generally seen through the prism of submission and control. This is apparent in the two Latin words for the act: irrumare (to penetrate orally), and fellare (to be penetrated orally). Under this system, it was considered to be abhorrent for a male to be in any way penetrated (be controlled) by another person of lower social standing during sex. This same logic also allowed a man to receive fellatio from a woman or another man of lower social status (such as a slave or debtor), because the man would be directing the actions of the person of lower rank. The Romans regarded oral sex as being far more shameful than, for example, anal sex — known practitioners were supposed to have foul breath and were often unwelcome as guests at a dinner table. The women of Lesbos, ironically enough, were believed to have introduced the practice of fellatio, and it is said that they used to whiten their lips as though with semen.
So, whereas in Greece, where there was a tendency to see the person "performing" oral sex as active and the "receiving" party as passive, in Roman times fellatio and cunnilingus were perceived to be a passive and therefore shameful act for any man to perform, and oral sex between members of low social standing groups was considered superfluous and was often viewed as taboo. Therefore performing any type of oral sex was considered to be a passive (as in submissive) act while receiving oral sex was viewed as an active (as in controlling) act.  The practice was taboo for public health reasons, as well. In Rome, the genitals were considered to be unclean. Oral sex was thought to make the mouth dirty, and (ultimately) to present a public health risk.
Today performing/giving and receiving/getting oral sex are both considered both active and passive. Gay men seem to consider performing oral sex active, while straight women seem often to be treated as the passive partner when doing so. Oral sex is still nominally illegal in some jurisdictions (often under sodomy laws) in western countries and is expressly illegal in many other nations. However, these laws often expressly target same-sex couples or are used only against same-sex couples. Matters of definition entered prominently into the 1998 Lewinsky scandal, when then-U.S. President Bill Clinton stated publicly "I did not have sexual relations with that woman" even though he had repeatedly received oral sex from Monica Lewinsky. In fact, in a telephone conversation Lewinsky had after her encounter with Clinton, when asked if they had had sex, Lewinsky replied, "No, we just fooled around."
Oral sex can be a method of contraception (birth control) since pregnancy is impossible unless semen enters the vagina. Many people choose oral or anal sex as an alternative to intercourse for this reason (although it is possible for semen to enter the vagina during anal sex).
While some people are turned off by the idea, many others find oral sex very pleasurable and use it even when avoiding pregnancy is not a concern, sometimes as part of foreplay. Not only are the sexual organs sensitive and well supplied with nerve endings, the same is true of the mouth, tongue and lips, so the pleasures of oral sex are not limited to the person on the receiving end.
The common term blowjob is misleading, as blowing is not normally part of fellatio. A common technique is to take the glans of the erect penis in the mouth while rhythmically caressing the rest of the penis with the hands; the testes and the shaft of the penis can also be licked with the tongue. Suction is often used to increase the pressure and friction exerted on the penis. (When the testes are taken into the mouth, the act is sometimes known as "teabagging" due to its supposed similarity to the action of dipping a teabag).
Depending on the preference of the participants, this stimulation may produce an orgasm and ejaculation of semen which may or may not be swallowed. It should be noted that intense sexual arousal of the penis will often produce a small quantity of fluid from the urethra at the tip of the penis, commonly referred to as "precum". This fluid is produced by the Cowper's glands and is not semen (and has been demonstrated to contain no sperm). The purpose of this bead may be to cleanse the urethra and prepare it for the ejaculation of semen that may soon follow, as well as to provide lubrication for the foreskin and glans during foreplay and intercourse.
Many males regard the reception of oral sex as an ego boost, believing that such an act is a form of dominance over their sexual partner. In addition, if ejaculation is reached by the male, the swallowing of the resulting semen by the fellator is regarded as preferable by the ejaculator. However, if the partner is unwilling to swallow or allow the semen into their mouths (and spit it back out), a common variant consists of fallatee ejaculating over the fallators face/body. The slang term for this is known as a "facial."
A rarer technique is deep throat, popularized by a pornographic movie of the same name. It involves taking the entire erect penis in the mouth, the shaft bending slightly to allow the head of the penis to slide partially down the throat of the fellator.
A small percentage of men have the penis length and/or body flexibility to fellate themselves, or to autofellate.
Irrumatio is similar to fellatio, but involves the penetration of the mouth as a passive orifice, making the involvement of the person thus penetrated a passive one, with little control over the proceedings. A slang term for this is "face fucking."
The vulva and especially the clitoris are kissed or licked with the tongue. The entrance to the vagina can also be stimulated by the tongue. For some women cunnilingus, especially that received from partners with a tounge piercings, is much more effective than intercourse in developing their orgasm.
It is possible to freshen one's breath (using breath mints, for instance), in such a way that the vulva does feel an impression of freshness.  Humming is also possible, pleasurable and efficient (in terms of energy expenditure per orgasm) during cunnilingus, as in fellatio.
The receiver of fellatio or cunnilingus may find it pleasurable if the giver hums or sings at the same time. This in effect makes the giver's mouth into a vibrator, though unlike a mechanical vibrator the mouth has built-in lubricant (saliva)
While the anus is technically not a genital or reproductive organ, anilingus—the practice of oral stimulation of the anus, also known as "rimming" or "a rim job" or "tossing someone's salad"—is often classified under the rubric of oral sex. Performing anilingus while manually stimulating the male member is also referred to as a "rusty trombone."
The Blumpkin is a form of fellatio combining genital stimulation with anal stimulation. It has been suggested that the term Blumpkin may also refer to a form of fellatio where the receiver is fellated while sitting on a toilet or latrine of some sort and releasing a bowel movement.
Semen contains water, small amounts of salt, protein and fructose sugar. It is slightly alkaline which causes some to find the taste bitter or brackish, but neither male nor female sexual bodily fluids are in themselves harmful to a sexual partner, apart from concerns of STDs (separately discussed below).
Urban legend sometimes describes semen as nutritious and a good source of protein; however, this is at best minimally true. Semen contains only a small amount of protein and in any case is typically only available in small quantities during oral sex. However the results of at least one study  suggest that semen may contain hormones or hormone-affecting agents which can improve mood and lessen the incidence or severity of depression when absorbed via a mucous membrane (such as in the mouth, vagina or anus). The same may hold for semen orally ingested and digested.
Partners should be careful not to bite or scratch the genitalia with the teeth harder than desired. It is also possible for the inside of the mouth to be bruised by the penis if oral sex is too rough. People with Temporomandibular Joint Disorder may find that giving oral sex is uncomfortable.
Sexually transmitted diseases
Although oral sex is widely considered to be safer than vaginal or anal sex, chlamydia, human papillomavirus (HPV), gonorrhea, herpes, hepatitis (multiple strains), and other sexually transmitted diseases (STDs) — including HIV — can be transmitted through oral sex. It should be stressed that any kind of direct contact with body fluids of a person infected with HIV (the virus that causes AIDS) should be avoided.
Furthermore, oral sex should be avoided when the either partner has wounds or open sores on the genitals or mouth, or bleeding gums in the mouth, or has recently brushed, flossed, undergone dental work, or eaten crunchy foods such as potato chips, all of which can cause small scratches in the lining of the mouth. These wounds, even when they are microscopic, increase the chances of contracting STDs which can be transmitted orally under these conditions. Such contact can also lead to more mundane infections from common bacteria and viruses found in, around and secreted from the genital regions. Because of this, many medical professionals advise the use of condoms in the performance of fellatio (flavoured condoms are available for this purpose) and the use of plastic or latex sheets (dental dams or ordinary plastic wrap) for cunnilingus.
If air is blown strongly into the vagina of a pregnant woman, there is an extremely slight chance that an air bubble can enter the women's circulatory system through the placenta and cause an embolism which could lead to death. There are very few reports of this in the medical literature, and it is regarded as a rare condition. However, lightly blowing onto the vulva is both pleasurable and perfectly safe.
There are many words describing oral sex, including euphemisms and slang. The jargon word fellatio comes from the Latin term fellare which means "to suck." A common slang term for giving oral sex to either a man or woman is giving head to or going down on someone (although in some regions, "giving head" refers only to fellatio). The term gamahuche is also sometimes used. Performed on the genitals of a man, oral sex is technically called fellatio, but common slang includes terms such as blowjob, cocksucking, "giving head", and in the UK, french, as well as many other slang terms. Performed on the genitals of a woman, it is known in the jargon of sexology as cunnilingus or in common English, cunt licking. There are far fewer slang terms for cunnilingus than there are for fellatio but they include: eating out, sugartime, muff-diving, carpet munching, pussy eating, giving a tornado, having a box lunch, dining at the Y, eating pussy, and giving a tongue bath. "Cocksucker" is considered a severe insult by some, and in the U.S. is one of the "seven dirty words".
- Kaiser RT. Air embolism death of a pregnant woman secondary to orogenital sex. Acad Emerg Med. 1994 Nov-Dec;1(6):555–8.
- Hill BF, Jones JS. Venous air embolism following orogenital sex during pregnancy. Am J Emerg Med. 1993 Mar;11(2):155–7.