I never thought i'd never be that girl.
I had s** with my assistant manager (who has a girlfriend). Everybody at work now knows this. Then I had s** with my manager (who I REALLY like but who also has a long-term girlfriend). No body at work knows this. Then I got hammered and got with my best friend/sisters boyfriend. No body knows this. And that last one is really the only one I truely regret and would take it back.
I never thought i'd be the girl who gets with people who are already taken. But my assistant manager was a really close friend but a mistake (although I wouldn't take it back - he was my first). Then; I am/was obsessed with my manager and I couldn't care less if he has a girlfriend. I know that sounds bad but I can't help it. And I don't feel bad for her because she did it to his wife.
The last 4 or 5 guys i've been involved with have had girlfriends. It amazes me that this percentage of guys cheat with such little persuasion.