If there was proof, I would sit down with him, confront him and ask what is going, and why. Then I would likely pack up, and remove myself from the situation.
Cheating is a number of things to the one who is cheated on: it is a betrayal of vows, it is lying to your spouse's face, it is dangerous in that he could be bringing an STD or worse into the relationship and it says a great deal about the integrity of the cheater.
Who needs it?!?!