The only way I would is in the context of a swingers club/party where I knew it was ok (I have started to get into this a bit, out of curiosity and, well, lack of sex!

), or if he's without any doubt legally separated (this has happened before too although we didn't actually have sex. I was too afraid of getting pregnant at the time - it was before I got "fixed"...)
I might also be in the unfortunate situation of being with someone not knowing that he was married. I hope that doesn't happen, but it sometimes does. One guy did come on to me once and I didn't go for it (something about him turned me off - besides I thought he was too old for me), then later I found out that he was married, and was very glad I didn't!