Pregnant girlfriend caught cheating - "I caught them kissing..."

I have been with my fiancee for 5 years,she is 12 weeks pregnant (which she plans to terminate) and in the last few months a "new friend" appears on the scene(work colleague) and she has been going out and getting blind drunk with him.

Our relationship has always been strong and we have never had any problems. I accused her of messing around with the the guy to which I was shot back down,she denied anything has happened but the body language and manner was wrong !

My gut feeling was telling me different. Then on Sat afternoon I caught them kissing and after talking about it, it has apparently happened before.

What on earth do I do? She wants a second chance or do I just leave ? I'm gutted.