When I was in a serious relationship I would never. For example, this guy tried to get me to date him instead of my boyfriend. I was tempted because I've known him for a loooong time and always thought he was hot but I told him to get lost. And I was so glad because when it was over between me and my boyfriend, I went after this same guy and he tried to play me with another girl! Should have seen that one coming in retrospect, eh?
However, a couple months back I was in a relationship with this guy. I don't even know why...I was bored & lonely. But I did cheat on him...and broke up with him a little after that. Strangely, I never felt bad or guilty about it...I just figure it's because I had so little feelings for him. I feel worse about it now, because obviously it's a sleazy thing to do.