A 2009 study found that heterosexual male fat admirers preferred females that were clinically overweight and rated both overweight and obese women more positively than slighter individuals. Snowballing or snowdropping is the human sexual practice in which one person takes someone else semen into his or her mouth and then passes it to the mouth of the other, usually through kissing. It is also followed by dedicated industry publications and trade groups as well as the mainstream press, private organizations , government agencies, and political organizations. Concerted deployed sexual rejection exercised on the partner can be an aspect of Dominance and Submission as well. Women with women Main article: Lesbianism in erotica: Lesbianism in contemporary pornography Lesbian or girl-girl pornography features women engaged in sex together, but is typically aimed at a male audience. Women who perform in lesbian pornography often do not consider themselves lesbian or bisexual. The consensual submission of the submissive is sometimes demonstrated to others by symbols indicating his/her belonging to the dominant, such as wearing a collar, special tattoos, piercings, a very short haircut or a bald head. If one person is penetrated by two objects, it is generically called double penetration milf fuck . The term is typically used by the cinematographer within the narrative framework of a pornographic gay hentai film, and since the 1970s has become a leitmotif of the hardcore genre. Felching is a sexual practice involving the act of orally sucking semen out of the vagina or anus of one partner. Group sex also occurs in populations of non-human animals such as bonobo apes and chimpanzees. Foot fetishism has been defined as a pronounced sexual interest in the feet or footwear. Types of anal sex can also be a part of lesbian sexual practices. A bisexual threesome may involve a man having sex with a man and a woman, the woman having sex with a man and a woman, or all three having sex with videos gays each other. Reality and voyeur pornography, animated videos, and legally prohibited acts also influence the classification of pornography.