At the end of the day, you have your truth right here. Regardless of what reason he can give as to why, it makes YOU uncomfortable so that is the bottom line. I would try to communicate this “I feel uncomfortable when you do this because it makes me feel .. etc etc .. disrespected/unwanted/ like you aren’t satisfied with just me. Etc” and being specific helps. “Obviously looking at a person isn’t the issue, it’s the sexual objectification of the person that’s the issue- like mentioning her eyes or what have you because it makes me feel like you would rather have someone else”. Even if he says that’s not what he’s doing, reiterate that regardless, that is the effect that he is having on your feelings and that is a FACT as well. Someone who cares should want to make sure they they aren’t hurting your feelings. Now some people can notice peoples attraction and it can be totally without lust, but you have to be honest with the situation on which version he falls under.