So I've been friends with this guy for several years now. He was in an open relationship when I became single a year ago and he told me he had always wondered what it would be like to be with me. Everything started there. We talked about getting together and having sex a few different times, but we never actually did. When his relationship became more serious, we decided to just be friends and nothing more.

We have been texting each other constantly for almost a year now and talking on the phone. We pretty much tell each other everything. We've even had little arguments and we stop talking for a week or so, and then we start again. We were always friends and had always talked to each other about stuff, so I didn't think it was wrong.

I always wondered if he had any feelings for me because of some of the things he would say to me, but I always just ignored it and didn't think much of it. Lately, he's been telling me he's been very stressed and has a lot of things on his mind. Then, over the weekend, he was drunk and texted me, and I was also drunk. He told me he liked to pretend he was special and important to me. Then we started talking about sex and how we had never gone through with it and how he wished we had. Then he asked me if I wanted to be his girlfriend and get married. I was shocked. I reminded him he already has a girlfriend. Then he continued to tell me how badly he wants me. Then asks me if I want to date him. I remind him again he has a girlfriend. He tells me he wants to see if things would work between us. He pretty much tells me he will leave her for me. I know he's not happy with her and not satisfied.

I don't know what to do. I like him and if he was single, we'd be dating by now. I think he knows that. I haven't talked to him since that conversation. It's been 2 days now. What should I do? Should I just let this go or should I pursue it and see where it goes?

OKAY SO YOU tell him that IF AND WHEN he is broke up with her that you just may well be his girlfriend if still interested but as long as he is with her your hands are tied.
Don't pursue it or anything just tell him like it is when the subject comes up BY HIM again.

well if he was drunk he would have had no clue what he was doing

Exactly, we have no idea if he even remembers what he said to you.

You don't do anything because he has a girlfriend. If he didn't like her anymore, he should break up with her, that's his business. If he really liked you, then he wouldn't be with his current girlfriend. The fact that he's still with her means that there's still something holding him back, so once again he's off limits.