However the gay boy porn genre remains a very small proportion of the pornographic DVD market; for example at porn retailer HotMovies. Strictly speaking, bondage means binding the partner by tying their appendages together; for example, by the use of handcuffs or by lashing their arms to an object. Going back at least as far as the Christy twins in the 1970s, depictions of incest and particularly incest between twins has been a feature of gay pornography. Emetophilia pornography involving vomiting and gagging. The terms can be complimentary or pejorative. Bareback sex is physical sexual activity, especially sexual penetration, without the use of a condom. Performers are usually clean-shaven, thinner and more effeminate than ones in mainstream gay pornography. Interracial pornographic films have experienced an explosion in popularity, becoming one of the fastest-growing and biggest-selling genres. Such explicit sexual interaction is seen only rarely in public play spaces, and it is sometimes specifically banned by the rules of a party or playspace. The name of the practice, gay hentai when it is done in a repeated in-and-out motion, is derived from its passing resemblance to the dipping of a tea bag into a cup of hot water as a method of brewing tea. This is commonly called double anal or double stuffing. Group sex can occur between people of all sexual orientations and lesbian milfs genders. Using the term shemale for a transsexual woman often implies that she is working in the sex trade. The term soft crush refers to the more common fetish surrounding videos involving inanimate objects or small invertebrates being crushed, while the term hard crush refers to such videos involving larger animals with vertabrae, and arguably more pain-susceptible animals . Kinbaku means tight binding Kinbaku-bi which literally means the beauty of tight binding. Sadomasochism does not imply enjoyment through causing or receiving pain in other situations . Shibari is a Japanese word that literally means to tie or to bind.