Can't really say anymore than what's been said above, your enabling him to cheat if you sleep with him, I think maybe he sounds like he'll cheat no matter what, I don't know full details obviously. At the end of the day, yes your single and he isn't but you know he's not single so as I see it your just as guilty of cheating if you go ahead with it, If he was just someone random you met on a night out, or someone you knew but never knew he was married then I would say different, but the thing is you do know. I think you know that, but I think deep down you just want to be with him and your looking for someone to say go ahead and do it, it's fine.
And if you want then I'll tell you that, if you can live with yourself for possibly breaking up a marriage, or a family if things were to come out then do it. If you can live with the fact that if it comes out, your friends and family will probably find out, then do it.
Maybe you don't have feelings for him now, but who knows in the future?? Do you want to possible end up 2nd best to his wife all the time, someone he'll hide away, hoping that his naughty little secret doesn't come out... and possibly without a second thought he'll drop you like a lead ballon if he thinks he's about to be caught out?
It's not nice to be treated like this, and it isn't something I'd wish on you. Another thing, is the old phrase, "once a cheater, always a cheater", there is a high chance you might not be the only one, or if things ever did progress and you both ended up together (who knows what the future holds) then could you ever fully trust him?
If your happy with all that and you accept all the risks and dangers and feelings of EVERYONE involved then go ahead, do it, if it's what you want. I'm not judging you, I just hope you think about it very long and hard..... I've made mistakes in life, and now I wish i'd just sat down and thought about it, with my head on my shoulders and not my head on something else.
What ever you decision, I wish you the best.