I've been on both sides of this (I'm a 28 year old guy).
On the guy's side - I wasn't as extreme, but recently I pretty much lost a friend because I told her how I felt and she clearly couldn't deal. We weren't that close, but we'd spoken fairly regularly, and I started to see a lot in common, and thought she felt the same way (partially because I was told by mutual friends we should get together. Assholes -_-). Anyway, after getting shot down I tried to remain friends, but she kept blowing me off to the point where I got angry. See, it stung a bit being shot down, it always does, but what can hurt the most is losing a friend without understanding why (she never did tell me she felt uncomfortable, I just had to assume, despite her telling me she did want to remain friends). From what I can tell, you clearly don't think of him as much of one, but maybe he thought differently. It happens. If he sits quietly in the corner when you and your friends hang out, maybe that's just how he deals with people, and maybe he assumes that if he's close with them then he must be close with you. I don't know really, but if he knows you have a boyfriend, and knows you're not interested, he should back off, but then if you don't want him as a friend, just tell him. It'll make things easier in the long run for you both. If you don't mind my asking, how exactly do you know this person?
edit: Also, as far as I know she's single, and I never mentioned being horny to her... and I never go for girls in a relationship... OK, I wasn't much like this guy beyond getting shot down, but the advice does still work!
On your side - I am blind to when a girl likes me, let me say that now. One of my favorite things about the present is that it's now acceptable for a woman to ask a man out, because otherwise I think I might have had 2 girlfriends in my entire life. That said, there have been women who liked me and I didn't realize, and there have been some that told me how they felt and I was honest with. At first things were a bit awkward, but it worked out well in the long run, mostly.
I did once have someone who could not accept that I didn't want them. We had dated a bit, fooled around, but I lost interest. Afterwards she would regularly try to seduce me. I told her it would never happen again, but in the end I had to ignore her and remove her from my life. It sucked, but ultimately it was the right thing to do both for myself, and for her, because now she has the chance to find someone new (and if she has to think I'm an asshole to do so, that's fine by me as well I guess).
