I actually had this happen recently. I was talking to a female friend of mine who was telling me about some *ahem* interactions she would occasionally have with a female friend of hers. Jokingly, I told her that I had the internet for that sort of thing, and didn't need to hear it from my friends.
She said she could probably hook me up, and I was like "No, really, it's ok..." Among other things, I'm not an extremely promiscuous person, and for another, I didn't know the girl's name. She's an attractive enough person, but there's this kind of "OMG WEIRD" thing that comes up at the prospect of possible sex with someone I barely know.
In any case, she asked me if I was ****ing my (male) roommate in response, which made me LOL. If by "****ing* you mean "Playing Dawn of War over a LAN", then sure! Otherwise, not so much.
Another friend of mine commented to me once that "Now that I know you like girls, we have to get you a girlfriend." Apparently, since she'd never seen me with a girl, she wasn't sure that I liked them.
Apparently there's an expectation that heterosexual males be relatively open to sex with women they don't know very well, based on looks. Not fitting that seems to imply to people that I'm gay, even though I'm not.