MTS, you sound very much like me 12 years ago.
There was a girl i was close to when my marriage was on the rocks. We were close friends, in the end nothing more happened but i did consider taking it further. I finished with my ex and decided to go it alone for a while to get my head straight, this was the best thing for me.
If you have split with your wife and it's over there is nothing wrong with moving on. As i have said calling it an affair is the JW mindset. If however you need people's advice on it, perhaps you are not ready and just on the rebound, or perhaps you think finding a new woman is going to solve all your problems.
There is a whole new world out there post JW's and post a bad marriage, don't make any rash decisions if you're unsure. If you're anything like i was you need a bit of time to sort your head out, in my case this was a couple of years.
Paul
Edit: If it's just a one night stand you're after what's the big deal? why take advice on it? Just go out on a Friday or Saturday night to any nightclub in Britain and it's there on the plate if you're up for it. From my own experience, believe me i have partied hard, it isn't going to make you feel better about you and your ex. Yes you've had a good night and maybe you need to get it out of your system. But waking up next to a stranger, who's name you have forgotten or didn't even ask, doesn't really make things better. In my case it was more about catching up with a lost youth than anything else.