I think people who don't have sex before they're married can have successful marriages, but JS does make them look bad! Bottom line, she was immature. And stupid. I don't think she ever took her marriage vows seriously.
Once the marriage was already falling apart, I think the fact that Jessica was a virgin when they married and Nick really wasn't - I heard he even tapped some groupie ass - could have caused her to want to sort of even up the score with him by putting a few more notches on her belt.