It is mutual consent that makes a clear legal and ethical distinction between BDSM and such crimes as sexual assault or domestic violence. This technique may include the use of a simulacrum, or artificial vagina. It may involve sexual attraction to women who are pregnant or appear pregnant, attraction to lactation, or attraction to particular stages of pregnancy such as impregnation or childbirth. The interaction between tops and bottoms—where physical or mental control of the bottom is surrendered to the top—is sometimes known as power exchange, whether in the context of an encounter or a relationship. Mutual masturbation can be practiced by males or females in pairs or groups with or without actually touching another person as indicated by the following examples of contact versus non-contact scenarios: Non-contact mutual masturbation Two people masturbating in the presence of each other but not touching. Making a pearl necklace is one of the activities that sex workers use as safe sex alternatives for people who refuse to wear condoms. Men and women gay cock may masturbate until they are close to orgasm, stop for a while to reduce excitement, and then resume masturbating. Other possibilities are possible, such as a man having sex with two women who are having sex with hentai videos each other. This category also includes pornographic home movies. Going back at least as far naked milf as the Christy twins in the 1970s, depictions of incest and particularly incest between twins has been a feature of gay pornography. In gay pornographic movies, actors who identify themselves as heterosexual, but who nevertheless perform explicit sexual acts with other males on film do not face the same stigmas as the mainstream acting counterparts and indeed can rise quickly to being featured actors. Teacher and Student or Boss and Secretary pornography involves sexualized situations which take place between an authority figure, such as a teacher or boss, and an underling, such as a student or secretary, often fake.