What should I do?

So my boyfriend has a female friend that is lesbian but she always calls him daddy and he sometimes calls her babe, I know it’s a joke they have but I’m not comfortable with it. I know they don’t have a thing at all and I trust him with my whole heart but I hate that they call eachother that. Those are names only he and I should have. When I talked to him about it he gets a bit upset and tells me over and over that it’s a joke and reminds me that she is into girls and that she supports our relationship like it’s her religion. What do I do? I know it’s a joke but like I said I really don’t like it and he won’t dont anything about it.

Update: I talked to him about it and he says he understands where I’m coming from and that I’m right about it. He said he would stop calling her that and have her stop calling him that.