Uncomfortable dating situation. Please advise.?

I don't have any good advice, because personally I'd be furious if my bf did this to me. I admire you women who accept their bf's female friends regardless of their relationship. If my bf had a crush on a girl back before he met me and then she began flirting with him incessantly during our relationship, you can bet I'd be talking to her AND him about it. What I want to know is why he's holding on to her so hard - does he enjoy her attention that much? He sees that it's bothering you but he's choosing her attention over your peace of mind. Why wouldn't he just tell her to back off?