Marriage is BS.
Screwing a married chick can have various monetary and legal ramifications for both her and her husband, so doing that has the possibility of making you into a huge jerk.
Screwing an engaged chick does not have any legal concequences. Worst case scenario is that their relationship is ruined. Best case is that nobody finds out.
I remember when I did this. It was about a year ago. Now she's married and happy, and I am happy that I had a good night with her. We've gone on with our lives.
So go for it. Who cares?