I concur with all of the above. Don't make excuses for him. There shouldn't be any contact with this woman, period. Surely there are other cute kids out there, including possibly, your own? If you see yourself spending the rest of your life with him, I would sit down pronto and make sure he satisfies your concerns about fidelity. Infidelity almost invariably means that a person is not happy in their relationship. I have been lucky in my own life to not experience it but I know that I would immediately end a relationship if my partner was cheating on me; knowing full well that I would be resentful and distrustful for a long time and that this would ultimately poison the relationship, I would not make any attempts to hold on to her, as painful as that would be. What makes this easy is the fact that I don't subscribe to the notion that there is "one" person out there for us. Rather, that there are plenty of perfectly qualified fish in the sea.