Generally, interest differs between people, and gender depends on sexual orientation. Mammary intercourse is said by Alex 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 Charnels of 1903. This technique may include the use of a simulacrum, or artificial vagina. In recent years, Japanese films of this genre have become more common in the USA and Europe although it still remains a small, fetish-oriented part of the adult film industry. A celebrity may allow such a recording momsex to surface willingly, as part of a publicity stunt. Partners can engage in penetrative sex, known as full swap, or choose to soft swap in which they engage only in non-penetrative sex. This category also includes pornographic home movies. When sexual activity is centered on one of the threesome, then the activity may be described as a gang bang of the person. Participants usually derive pleasure from this, even though many of the practices—such as inflicting pain or humiliation or being restrained—would be unpleasant under other circumstances. Clothed female, naked male is a genre of erotica featuring one or more nude men and one or more clothed women. It is mutual consent that makes a clear legal and ethical distinction between BDSM and such crimes as sexual assault or domestic violence. The term hentai is never used in this context in Japanese. Another variant of reality pornography consists hentai porn of normal gay boy porn couples that are filmed by professionals, in which case the only obvious distinction from amateur pornography is the higher quality of production, filming and editing. 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. Pornographic films are generally classified into sub-genres which describe the sexual fantasy which the film and actors attempt to create. Male masturbation may differ between males who have been circumcised and those who have not.