If your voice doesn't shatter glass when you're under duress, proclaim that you're a man, and tell him everyone in prison will know he's a fag (be sure to use that term) when he gets caught. If you're convincing, you might get a thorough beating, but unless he got around to checking you might get away unraped. Maybe. On the other hand, rapists love guns (it goes hand in hand with the power trip), so pretty much anything you say could get you shot.
If he's unarmed, nonverbal communication would probably be your best bet. Calmly sticking a finger through his eye or ripping off an ear would probably do it. (For some reason, most people being assaulted tend to shy away from mutilation. It's like coloring outside the lines or something.)