Biological anthropologist Helen B. Cuckolding can also be mixed with other non-monogamous relationship arrangements with which it has substantial overlap such as Swinging, wife-swapping, open relationships, and polyamory, etc. The glans itself may widen and lengthen momsex as the stimulation continues, becoming slightly darker in colour, while the gliding action of the foreskin reduces friction. The term bondage describes the practice of Physical restraining. Some anthropologists and sociobiologists believe that breast fetishism derives from the breasts similarity to buttocks, but instead provide sexual attraction from the front of the body. If one person is penetrated by two objects, it is generically called double penetration . Some may keep their hand stationary while pumping into it with pelvic thrusts in order to simulate the motions of sexual intercourse. Internal ejaculation shots are a comparatively recent development in pornography; they are not found in early pornographic films. Testosterone discourages fat storage in these areas. Femdom features BDSM scenes in which the dominant partner is female. A survey 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. When used as an alternative to penile-vaginal penetration, the goal may be to preserve virginity or to prevent pregnancy. 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. Hardcore pornography is pornography that depicts penetration or extreme fetish acts, gay twink porn or both. Transsexual porn star Wendy Williams has stated that transsexual Porn is classified as Straight Specialty. Oral sex pornography frequently features a person performing 3d hentai fellatio, and less frequently cunnilingus. A study of 164 males in Finland from two SM clubs found that 18.2% had engaged in coprophilia; 3% as a sadist, 6.1% as a masochist, and 9.1% as both. 18% of heterosexuals and 17% of homosexuals in the study pool had tried coprophilia, showing no statistically significant difference between heterosexuals and homosexuals.