Dude... Please just save yourself. I agree with sarahsweets, the main problem is not whose baby it is BUT she cheats on you. Your relationship has already gone downhill, leave before you got dragged deeper, hurt and lose more than you have right now. Use your logic, not your feelings. She will drag you down. Yes it will be painful to leave and move on at this moment, but the outcome in the future would be better.

But if you really want to keep her, that's your choice and we can't force you not to. If you really do, prepare yourself to suffer more than you currently endure.... and she might probably leave you someday for that (or another) guy.