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Anal sex or anal intercourse is generally the insertion and thrusting of the erect penis into a person's anus, or anus and rectum, for sexual pleasure.〔(【引用サイトリンク】publisher=WebMD )〕〔See (pages 270–271 ) for anal sex information, and (page 118 ) for information about the clitoris. 〕 Other forms of anal sex include fingering, the use of sex toys for anal penetration, oral sex performed on the anus (anilingus), and pegging.〔 Though the term ''anal sex'' most commonly means penile-anal penetration,〔〔〔 sources sometimes use the term ''anal intercourse'' to refer exclusively to penile-anal penetration, and ''anal sex'' to refer to any form of anal sexual activity, especially between pairings as opposed to anal masturbation.〔 While anal sex is commonly associated with male homosexuality, research shows that not all gay males engage in anal sex and that it is not uncommon in heterosexual relationships.〔 Types of anal sex can also be a part of lesbian sexual practices. People may experience pleasure from anal sex by stimulation of the anal nerve endings, and orgasm may be achieved through anal penetration – by indirect stimulation of the prostate in men, indirect stimulation of the clitoris or an area of the vagina associated with the G-spot in women, and other sensory nerves (especially the pudendal nerve).〔〔〔See (page 3 ) for women preferring anal sex to vaginal sex, and (page 15 ) for reaching orgasm through indirect stimulation of the G-spot. 〕 However, people may also find anal sex painful, sometimes extremely so, which may be primarily due to psychological factors in some cases. As with most forms of sexual activity, anal sex participants risk contracting sexually transmitted infections (STIs/STDs). Anal sex is considered a high-risk sexual practice because of the vulnerability of the anus and rectum. The anal and rectal tissues are delicate and do not provide natural lubrication, so they can easily tear and permit disease transmission, especially if lubricant is not used.〔〔 Anal sex without protection of a condom is considered the riskiest form of sexual activity,〔 and therefore health authorities such as the World Health Organization (WHO) recommend safe sex practices for anal sex.〔World Health Organization, Department of Reproductive Health and Research (Global strategy for the prevention and control of sexually transmitted infections: 2006–2015. Breaking the chain of transmission ), 2007, ISBN 978-92-4-156347-5〕 Strong views are often expressed about anal sex. It is controversial in various cultures, especially with regard to religious prohibitions. This is commonly due to prohibitions against anal sex among males or teachings about the procreative purpose of sexual activity.〔〔〔 It may be regarded as taboo or unnatural, and is a criminal offense in some countries, punishable by corporal or capital punishment; by contrast, people also regard anal sex as a natural and valid form of sexual activity that may be as fulfilling as other desired sexual expressions. They may regard it as an enhancing element of their sex lives or as their primary form of sexual activity.〔〔〔 ==Anatomy and stimulation== The abundance of nerve endings in the anal region and rectum can make anal sex pleasurable for men or women.〔〔〔 The internal and external sphincter muscles control the opening and closing of the anus; these muscles, which are sensitive membranes made up of many nerve endings, facilitate pleasure or pain during anal sex.〔〔 The ''Human Sexuality: An Encyclopedia'' states that "the inner third of the anal canal is less sensitive to touch than the outer two-thirds, but is more sensitive to pressure" and that "the rectum is a curved tube about eight or nine inches long and has the capacity, like the anus, to expand".〔 Research indicates that anal sex occurs significantly less frequently than other sexual behaviors,〔 but its association with dominance and submission, as well as taboo, makes it an appealing stimulus to people of all sexual orientations.〔 In addition to sexual penetration by the penis, people may use sex toys such as butt plugs or anal beads, engage in fingering, anilingus, pegging, anal masturbation or fisting for anal sexual activity, and different sex positions may also be included.〔 Fisting is the least practiced of the activities, partly because it is uncommon that people can relax enough to accommodate an object as big as a fist being inserted into the anus.〔 In a male receptive partner, being anally penetrated can produce a pleasurable sensation due to the inserted penis rubbing or brushing against the prostate (also known as the "male G-spot") through the anal wall.〔〔 This can result in pleasurable sensations and can lead to an orgasm in some cases.〔〔〔 Prostate stimulation can produce a "deeper" orgasm, sometimes described by men as more widespread and intense, longer-lasting, and allowing for greater feelings of ecstasy than orgasm elicited by penile stimulation only.〔〔 The prostate is located next to the rectum and is the larger, more developed male homologue (variation) to the female Skene's glands. However, though the experiences are different, male orgasms by penile stimulation are also centered in the prostate gland. It is also common, and likely most typical, that men will not reach orgasm as receptive partners solely from anal sex. General statistics indicate that 70–80% of women require direct clitoral stimulation to achieve orgasm.〔 The clitoris is composed of more than the externally visible glans (head). With its glans or body as a whole estimated to have around 8,000 sensory nerve endings,〔 the clitoris surrounds the vagina and urethra,〔 :〕 and may have a similar connection with the anus. The vagina is flanked on each side by the clitoral crura, the internal "legs" of the clitoris, which are highly sensitive and become engorged with blood when sexually aroused.〔 In addition to nerve endings present within the anus and rectum, women may find anal stimulation pleasurable due to indirect stimulation of these "legs". Indirect stimulation of the clitoris through anal penetration may also be caused by the shared sensory nerves, especially the pudendal nerve, which gives off the inferior anal nerves and divides into two terminal branches: the perineal nerve and the dorsal nerve of the clitoris.〔 The Gräfenberg spot, or G-spot, is a debated area of female anatomy, particularly among doctors and researchers,〔 :〕 but it is typically described as being located behind the female pubic bone surrounding the urethra and accessible through the anterior wall of the vagina; it and other areas of the vagina are considered to have tissue and nerves that are related to the clitoris.〔〔〔 Besides the shared anatomy of the aforementioned sensory nerves, orgasm by indirect stimulation of the clitoris or G-spot area through anal penetration is possible because of the close proximity between the vaginal cavity and the rectal cavity.〔〔 Achieving orgasm solely by anal stimulation is rare among women. Direct stimulation of the clitoris, a G-spot area, or both, during anal sex can help some women enjoy the activity and reach orgasm from it.〔〔 Stimulation from anal sex can additionally be affected by popular perception or portrayals of the activity, such as erotica or pornography. In pornography, anal sex is commonly portrayed as a desirable, painless routine that does not require personal lubricant; this can result in couples performing anal sex without care, and men and women believing that it is unusual for women, as receptive partners, to find discomfort or pain instead of pleasure from the activity.〔〔See (page 560 ) for effects of viewing pornography with regard to anal sex, and (pages 286–289 ) for anal sex as a birth control method. 〕〔 (Pdf. ) (Preview. )〕 By contrast, each person's sphincter muscles react to penetration differently, the anal sphincters have tissues that are more prone to tearing, and the anus and rectum, unlike the vagina, do not provide natural lubrication for sexual penetration. Researchers say adequate application of a personal lubricant, relaxation, and communication between sexual partners are crucial to avoid pain or damage to the anus or rectum.〔〔 Additionally, ensuring that the anal area is clean and the bowel is empty, for both aesthetics and practicality, may be desired by participants.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Anal sex」の詳細全文を読む スポンサード リンク
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