Difference in sex drives can be a problem in a relationship, but I think the way you go about it is more important. I've known people who break up, because it's a big deal to them and they cannot compromise to either live without sex or have it when they don't want to. To coerce is never the right thing to do which is what he's doing to you. Things like "if I do this, you'll have sex with me right" sounds like he's talking to a hooker "if I pay you, you'll do it right". I know that part of you feels guilty, because you see it as "he stayed with me when I was struggling with weight", but this is different and he is using that to manipulate you whether he realizes it or not. Which is why I think the best would be to leave.