This guy I like wants me to start blowing him.

I'm friends with this guy and have been since elementary. We're doing a project in one of our classes and we got pair together. We were in the library during our free period working. And out of the blue he ask me if I would be ok giving him head. I'm gay and a couple of years ago he figured​ I thought he was hot. I have a pool at my house and he comes over. I guess I'm not good at sneaky glances and one day he asked if I was gay and then asked if I liked him. Told him yes and he was cool with it, said it felt good to know someone liked him. This was years ago and we kinda ignore it, he would make a gay joke every once in a wild but nothing mean. He started asking for fashion advice and stuff cause he got a girlfriend. But back to today, I didn't know how to respond to it, literally started sweating. He said we could both get something out of it and that he's honestly curious. I was like sure I guess, my voice cracked, was sweating and my eyes immediately went to his Bludge. Wearing some somewhat tight white jeans so it was on full display. I guess I was getting hard cause he said we can do it right now. Go to somewhere private obviously. But I have a cold and my throat been hurting me so I told him we'd​ have to do it later. He suggested a handjob and I said sure. And he's gonna meet me at my house. But here a big problem. As I mentioned earlier, that girlfriend and him are still currently dating. Later I saw them together which I completely forgotten about her earlier. I've had a crush on this guy for years and I've seen him shirtless and pantless and I've had to control myself from going further. And now he's literally offering himself from me. The fact he tells me he's curious could mean something but there still dating so I have no idea what to do. His grades has always hated me cause she knows I like him. She's actually have been a bitch to me in past. Telling me to stop staring at him and stuff. Calling me a fag and to stay away and stuff. So I'm afraid of what she would do if she find out.