Haha, I've had a lot of gay guys hit on me. The first time was as wierd and awkward as you're describing, but now, it just rolls off my back. Here's what I do, it'll work for you, too. Tell him calmly that you're not gay, and you're sorry. Don't be mean, don't get pissed, and don't act like he's a digusting freak. Just say something along the lines of "look, I'm sorry, I'm not gay. I don't have a problem with you being gay, but please don't hit on me. It's uncomfortable, and it's just not gonna happen."