I am so sorry to hear this! This is too weird that he waited until you were post-op!!! Did he say "why" he cheated? Have you confronted him about this?
There are LOADS of WLS patients out there who have said that their spouses (or significant others) are worried that once the patients loses weight, they'll no longer be interested in their spouse. Sometimes this is true but, in most cases, it isn't.
Anyhoo, on for my guesses:
1) You obviously are looking better, feeling better and have maybe gained some self-confidence. You've jumped "off the shelf" and that might worry your husband. He might have been concerned that you'd eventually cheat on him, ask for a divorce, or something and wanted to "beat you to the punch", if that makes any sense.
2) OR he might be trying to "keep you in your place" because you ARE looking better, etc. In other words, maybe trying to make you feel unattractive (when in reality you are probably feeling more attractive than you have in a long time). Again, this would be in answer to his paranoia that you are going to leave him.
3) One last thought.....have you been paying the same attention to him as you did before surgery. Yup, I know there's a recovery time as there is for any surgery so that shouldn't even be taken into consideration. But does he feel left out? Has this become "all about you"? A lot of times, sudden weight loss can go to a person's head. While self-confidence is fantastic, self-absorption and self-centeredness is usually what drives spouses, friends & family away.
Anyhoo, don't know if I've hit on anything that might be helpful. Just trying to look at it from both sides here.
Please keep me posted on how this is going for you.