I agree with much in the previous post. Talking to your husband and therapy may both be a good idea to see how committed he is to the relationship. He's being an extremely selfish lover... if you can still call him lover under the circumstances. In my experience, sex reflects the relationship and I can't help but wonder if he is selfish in other aspects of your relationship.
There are several things of note here: more worrisome than just the sex is his lack of caring when you've brought up the issue. It may be that he cares and is uncomfortable confronting the situation or that he doesn't care that much. You need to find out which. If he cares and if you tell him you're worried about the marriage (if you want to go there), he'll talk or go to therapy. If he's just being a selfish jerk and you want more, this could be the beginning of the end. The cheating comment I find worrying as well: generally the people I know of who were paranoid about that were the type to do it. They were paranoid becuase that's what they would do! Big red flag if you ask me.
BTW, I love that you are confident. You can lose the weight for you, but I absolutely agree that you should not attribute the problems to your weight. Women can still be very attractive even when they're rounder than they'd like. :)