A person may be sexually penetrated multiple times simultaneously. A celebrity can fight the release in court to maintain deniability while still enjoying the career benefits. To avoid soreness from this resulting friction, some may prefer to use a personal lubricant, masturbation cream or saliva during masturbation. Real school shots or videos which are sexualized may violate privacy laws in some countries. Some people masturbate by using machines that simulate intercourse. Internal ejaculation shots are a comparatively recent development in pornography; they are not found in early pornographic films. 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. Such ejaculation can be done by mammary intercourse, or masturbation over the other person, or result from the withdrawal of the penis from a partner mouth as he is ejaculating following fellatio, irrumatio, or intercourse. Mutual masturbation, mutual manual stimulation of the genitals between partners, can be a substitute for sexual penetration. Unlike the usual power neutral gay videos relationships and play styles commonly followed by couples, activities and relationships within a BDSM context are often characterized by the participants taking on complementary, but unequal roles; thus, the idea of informed consent of both the partners becomes essential. In humans, females generally have more round and voluptuous buttocks, caused by estrogen that encourages the body to store fat in the buttocks, hips, and thighs. 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. Yuri : Featuring two grown adult women in a homosexual/lesbian relationship. Perhaps following the example set by gay pornography, they have become a popular subgenre within heterosexual pornography since the turn of the century, featuring both vaginal and anal ejaculations.