See Gay-for-pay. Sexologist Alfred Kind suggested that the buttocks is the primary sexual presentation site in primates. The term discipline describes psychological restraining, with the use of rules and punishment to control overt behavior. Examples include the Girls Gone Wild and Girls Who Like Girls Series. Mammary intercourse is said by Alex gay twinks Comfort to produce mutual orgasm in women with sensitive breasts , and it was one of the nine substitute exercises for penetrative sexual activities, as detailed in the Paradis lesbian hentai Charnels of 1903. Failure to honor a safeword is considered serious misconduct and could even change the sexual consent situation into a crime, depending on the relevant law, since the bottom has explicitly revoked his or her consent to any actions that follow the use of the safeword . In addition to mainstream pornography, the popularity of facials has led to creation of its own niche market, like video series that specialize in showing the act. They further momporn argue that setting a discrete line between safe and not-safe activities ideologically denies consenting adults the right to evaluate risks vs rewards for themselves; that some adults will be drawn to certain activities regardless of the risk; and that BDSM play—particularly higher-risk play or edgeplay—should be treated with the same regard as extreme sports, with both respect and the demand that practitioners educate themselves and practice the higher-risk activities to decrease risk. Transsexual porn star Wendy Williams has stated that transsexual porn is classified as Straight Specialty. Bareback sex is physical sexual activity, especially sexual penetration, without the use of a condom. The person sat upon may be in bondage, sexually submissive, or simply held down by the body-weight of the other person. Ass to mouth pornography shows the transfer of a performer penis from a second performer anus to that same second performer mouth in a single shot. BDSM actions can often take place during a specific period of time agreed to by both parties, referred to as play, a scene or a session.