Personally, I wouldn't want to get involved with someone that was married. The fantasy is all very nice to think about, but just imagine the consequences...

Firstly, in your case, what if the husband were to find out somehow? Secondly, there's the chance of attachment. As it'd be a fling, I expect she just wants to have some fun but it can be easy to form a relationship. I know if I were having a fling then I imagine I'd start to get feelings for the married man. And the other way round, you may expect just a good time but what if she decided she wanted more than just sex?

So to cut a long explanation short (lol, too late), I don't think it's worth the pain it can eventually cause. Besides, if I were the married person, I wouldn't want my husband to go cheat on me...