The only girl I ever dated that openly admitted to cheating on several previous boyfriends came very, very close to cheating on me before I figured out what was going on. In her case, I think it is simply the way that she handles boredom in relationships. Instead of dumping the guy (me), she found another one, trying to:
a) make me jealous, or
b) have a plan b in case I dumped her.
Turns out b) worked out really well for her.
After that experience, I'd say I have a hard time trusting a girl who admits to having cheated
several times. You can make a mistake and learn from it. If you make that mistake more than once, it is a habit.
(BTW, dark one hit the nail on the head about some of the circumstances that lead to such destructive behavior... I felt like he was describing my ex perfectly. Daddy issues, abused in her past, drug problems. Even though she seemed completely healthy by th time I met her, she'd use those excuses as a "reason" for her mistakes.)