"Behind Friends" is a gay porn video tag. In this scenario, the term "behind" is often used in the context of anal sex, which is common in homosexual relationships. So, "behind friends" refers to a gay porn video where two men engage in anal sex with each other, while being friends or partners. This tag indicates that the content features male-male sexual interactions and might involve activities such as rimming (oral stimulation of the anus), fingering, and other intimate acts before or after the anal intercourse