This is not uncommon in the gay community. I would guess that a lot of it is unreported because the victim may feel that they deserved it, the sex was probably what they were out looking for in the first place, and "did I drink too much or did somebody slip me a mickey?" "I remember flirting with the guy and didn't say NO before I passed out, so am I hurt because he raped me or because I missed it?"
Abuse occurs when you don't have the capacity, capability or free will to say no to everything that happens to your body. Even if you said Yes earlier, if you don't have the ability to say yes again when push comes to shove, you were raped. You had no choice. Even if you aren't sure in your mind whether you should call it rape or not, the guilt, shame and hurt are symptoms of abuse.
I hope this sheds some light,
"That which does not kill us, surely makes us stonger." - Neitsche