Teacher and Student or Boss and Secretary pornography involves sexualized situations which take place between an momporn authority figure, such as a teacher or boss, and an underling, such as a student or secretary, often fake. The person sat upon may be in bondage, sexually submissive, or simply held down by the body-weight of the other person. Especially popular in gay twink porn post-World War 2 Japanese culture. Amateur porn has been called one of the most profitable and long-lasting gay hentai genres of pornography. Saliva pornography involves salivation and saliva. 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. For uncircumcised males, stimulation of the penis in this way comes from the pumping of the foreskin, in which the foreskin is held and slid up and down over the glans, which depending on foreskin length, is completely or partially covered and then uncovered in a rapid motion. Male masturbation may differ between males who have been circumcised and those who have not. The effect is to give the viewer the sense that they are experiencing the sex acts that they are watching, as opposed to simply watching others as a voyeur. Crying pornography that features participating actors crying. The fundamental principles for the exercise of BDSM require that it should be performed with the informed consent of all involved parties. To tea bag is a slang term for the sexual act of a man placing his scrotum in the mouth of a willing sexual partner for pleasure or onto the face or head of another person. There is also some evidence that pornography can be used to treat voyeurism. The higher pay scale and profile within a production often leads to group scenes where a straight actor only tops. Transsexual porn star Wendy Williams has stated that transsexual porn is classified as Straight Specialty. In pornography, depictions of lesbian sex form a popular subgenre, directed toward a male heterosexual and female homosexual audience.