In my early twenties, there was a gay guy in our friend group. Two or three guys used to fuck this gay feller in the ass without a condom from time to time. The result: not only all of them got AIDS, but also two of the girls...
Alternatively, some fags like to latch onto girls in order to use them as baits to attract men, once everybody is drunk and things start to heat up between you and the girl, you may notice that the dude will hang around. My friend wasn't sure the guy was a fag and fell for this shit, thinking they were going to pull a train on the girl and, while he was fucking her from behind, all of the sudden her gay friend started to fondle his balls!