Particular areas and processes of the body that change during pregnancy may also become the focus of psychological investment, but nudity or sexual activity is not always essential, and in some cases actual pregnancy is not necessary to invoke arousal. The most common type of erotic spanking is administered on the bare buttocks, but can also be combined with bondage, in order to heighten sexual arousal and feelings of helplessness. 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 momsex clitoral/G-spot stimulation in women, and associated sensory nerves . Felching is a sexual practice involving the act of orally sucking semen out of the vagina or anus of one partner. Rainbow party A rainbow party is an urban legend spread from the early 2000s. Threesome can also refer to a love triangle, a three-way videos gays romantic relationship. The other person may do the same during or after. The act may be carried out face to face, or head to tail, with the woman recommended to be on top if she has smaller breasts. Such ejaculation can be done by mammary intercourse, or masturbation over the other person, or result from the withdrawal of the penis from a partner mouth as he is ejaculating following fellatio, irrumatio, or intercourse. Medical pornography involves sexualized medical, often fake, including gynecological pelvic examinations, rectal examinations, and pelvic or gynecological ultrasounds. Preggo pornography features pregnant performers and may include erotic lactation as a fetish. A survey lesbian hentai by the New York City Department of Health and Mental Hygiene found that heterosexual women are more likely to bareback when engaging in anal sex than homosexual and bisexual men. Diaper pornography involves performers wearing diapers, sometimes with an adult performer acting in the role of a caretaker. Sadism describes sexual pleasure derived by inflicting pain, degradation, humiliation on another person or causing another person to suffer.