My boyfriend of 2 years is studying with another girl. Should I be worried?

My boyfriend and I have been together for 2 years. We are both in our early 20s. 6 months after we started going out he started studying for his exams with one girl. Every time he takes a class, he finds a different girl to study with (and it is just one). He never studies with guys. I have confronted him about this but he just says that guys are unreliable and that he doesn't do it on purpose. I think the reason why he always studies with one girl is because he loves to flirt. When I met him he was very flirty but he has told me that he is not flirty now because he is in a relationship but I doubt it. I have seen him being flirty with girls a couple of times. I trust him in the sense that I don't think he is going to do anything with these girls but it bothers me that he would still flirt. Every time I try talking to him about it we just end up arguing because he says that according to his definition of "flirt" what he is doing is not being a flirt, that he is just being friendly but to me its more than that. This really bothers me and I don't know what to do about it. Every time I get mad I just shut myself down and this makes him really mad because I won't talk to him. At this point, I just really don't want to fight again over jealousy issues because I know how is going to end up. He would always make it seem as if I was the one over-reacting. What should I do? I really like him but I don't think a relationship can continue when you keep having the same fights because of jealousy and him being flirty with girls. Any advice? It would be very appreciated.
My boyfriend of 2 years is studying with another girl. Should I be worried?
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