Definition: Oral sex, or blowjob, are all that porn techniques requiring the use of mouth, tongue, teeth and throat to stimulate the penis and genitalia.
Cunnilingus refers to the kind of oral sex perfomed to women, while felattio refers to the blowjob performed to men. Analingus or black kiss refers to the oral stimulation of the anus, and, even though is one of the many kinds of oral sex, this term only refers to the oral stimulation of the genitalis. Oral sex is linked with preliminary, during or after sex erotic games. It can also be performed as a sexual act itself.
Oral sex is performed both in straight gay relationships. In straight cuples, oral sex is also considered as a way of not getting pregnant, because it's impossible if the semen doen't enter the pussy. Straight couples may also choose oral sex as an alternative to fuck because of this reason.
It's important to notice that oral sex is not an effective way to prevent from sexually transmitted diseases (STD), even though this way it's likely some of these won't infect so easily.
In the following link you can watch some free oral sex videos.
