I know I'll probably catch plenty of flak for this, but I'm gonna say it anyway.
I have slept with married (or otherwise "spoken for") men before, and I'm sure I'll do it again. Why? Because I want to. Because he wants to. Because there's not really a reason NOT to.
It's not like I go looking for married men. They find me. People in similar situations seem to have a knack for finding each other. I just keep my eyes open, that's all.
The question I'm sure that you're asking yourself is "Why? Why would she do that to another woman?" But see, that's the thing... I'm not doing anything to anyone. I have no loyalties to women I don't know. If someone's going to cheat, they're going to cheat. That's just how it is. I'm not doing the cheating for them. I'm not making them do anything they weren't already going to do.
People in committed relationships should understand that there's a profound probability that both parties involved will be tempted, usually several times over the course of their life. That's a pat of life. What they choose to do with that temptation is up to the involved individual.
So just remember ladies... treat your man right, and he probably won't end up with a girl like me. If you do, and he still does, then he's not worthy of you anyway, and you should be glad to be rid of him.