I'm not suggesting you have to associate with one in any fashion whatsoever, but OP has chosen to invite someone into her life who has been very clear about his wants and desires, and he has no obligation to change them for her or anyone else (unless eating meat becomes illegal, because then there is the legal obligation obviously).

She can always choose to terminate the relationship if his morality is a deal breaker for her, that is her right, but she can't force him to change, and if she wants the relationship to continue, then continuing to badger him about it when he has made his stance clear is a sure way to guarantee that it does not.

Much as it would be, I suspect, if the tables were turned, and he was continuously pressuring her into eating steak with him.