Oddly enough I've been asked by two guys. (Maybe I give off some kind of vibe, I don't know.)
The first guy I knew was gay and he made alot of jokes about it, then one day just falt out asked me. I said sure, and it was alright.
The second guy hasn't directly asked me, but basically told me he was bi, and that he had sucked off one of our other friends and liked it, and he wanted to do it some more. I left it open for him to ask me, but he didn't, so I'll just leave it at that. I'm not so sure if I would let him because I know him pretty well compared to the first guy.
But seriously? Turning down a free blowjob? I mean... Why would anyone do that?