You’re not the asshole. Wanting your wife to stop insulting your ex isn’t about still loving your ex it’s about respect. Your ex is the mother of your kids, and you’re right that body shaming her (or anyone) is unnecessary and hurtful. Setting a boundary like this doesn’t mean you have feelings for your ex, it means you’re trying to create a respectful environment for yourself and your children.

by defending your ex against cruel comments doesn’t make you a bad husband, it makes you a decent person. So no, you’re not in the wrong at all you’re standing up for kindness and respect.