Never, and I don't plan on doing so. I've never even chased somebody who's in a relationship because when someone's taken, they're out of bounds.
I just don't understand cheating. Whenever I've been unhappy in a relationship, I have talked to the other person about it. And just because you are unhappy in a relationship, that doesn't give you an excuse to cheat. I think it's the most cowardly thing a person can do. IMO, cheating is never justified. The worst kind of cheaters are the ones who do it when they're drunk and then try and say they "didn't know what they were doing".

I've been ridiculously drunk before now and I can still tell what's right and wrong.
Oh, and as a person who's been cheated on, let me tell anyone who's cheated that it hurts so much. It really affected me emotionally and mentally for a good 6 months after if happened. It really made me insecure and doubt myself. It's a cruel thing to do to somebody.