My girlfriend is pregnant as a result of cheating...what to do?

If she wasn't pregnant, would you stay with her? If the answer is yes, then stay. You will have to do some soul searching and make sure you are ready for being a Dad. I strongly suggest some counselling, both as a couple and by yourselves. Having a baby is so hard on a relationship even when there is a biological tie to both of you, you need to be prepared for the reality of the relationship coming second for a while, along with the intensity of caring for the baby.