A person's weight fluctuates constantly; plus to minus 5-10lbs isn't unusual. Chances are it wasn't the sex but the other associated activities that occur during courting that caused any weight gain. IE, eating together or post coital snacks or what not.
I can honestly think of no reason why sex in upon itself would cause weight gain.
The cheating... well that is a completely different issue. Are you still with her? If so, why?