With such a high level of betrayal, it's best to just walk out of the marriage. He gave up as soon as he invested himself into the other woman. Now she will have to stick around because they have a child on the way. Staying with him will only put so much unneeded stress upon you as well as deep insecurities and trust issues. Do yourself a huge favor by divorcing him. You deserve so much better. I promise you that another man will come into your life and truly love you.

That's low. And I don't mean to sound insensitive, but it's probably a good thing he didn't get you pregnant. If he had, you'd end up wanting him to still be a part of your kid's life, even though he'd cheated on you. Since you don't have a kid tying you together, divorce the son of a bitch, and start looking for someone better.