The random process of selecting a female during sexual intercourse when there is more than one female present. The females tend to be of such an attractiveness level that it is difficult to pick one or the other.
Bro 1: Dude, I had threesome with hot girl 1 and 2!
Bro 2: No way! How did you pick who to start with?
Bro 1: I played vaginal roulette.