It's normal to find other people attractive. It isn't OK to go on about them and to rub your gf nose in it. Especially when it's someone you know.
If he thinks it's 'normal' and OK - has he told his friend that he finds their gf sexually attractive and he likes her as a person too? Has he gone on about it to his friend?
At best he is very immature and thoughtless.
At worst he is doing this with the intention of hurting you and reducing your self confidence.
Either way, he isn't a good bf and I would honestly walk away.
Add to that the gaslighting and he comes across as being far from a catch. He doesn't treat you with respect, which is a basic requirement of a healthy relationship.
It's tempting to say reverse the tables. Pick a friend of his and keep going on about how sexy they are and what a lovely person. See if he likes it. And stop showing him that you're bothered by his comments. If he's doing it to wind you up it will stop being fun if you don't seem bothered.
HOWEVER I don't advocate playing games, and any time spent trying to teach him a lesson or get him in line is time wasted. You shouldn't have to treat a grown man how to be a decent human being.
Honestly, just walk away. And don't give him the satisfaction of telling him it's to di with his friend's gf and his negging. Just say you've gone off him, you don't find him attractive anymore.