i think i have to disagree with you guys. put yourselves in boyfriend's position... if ur girlfriend was going out with the one she defined "her best male friend", would you be perfectly ok with that? maybe some of you, but just the real confident ones.
i personally don't make a big deal about her going out with male friends, even though she used to like one of them, i often ask her to invite her friends over on saturday nights, but they hate me for staying with her (they like them since they were freshmen, now they are juniors), so they always refuse. not that i'm confident, i just wouldn't give a sh1t if she cheated on me with somebody i don't know (plus they are biggest AFCs). anyway, i'm talkin' about myself not you, i'm sorry
what i mean is, as soon as u get a chance to talk with her boyfriend ALONE (not to her presence), explain that you are very cool about them being together, but you noticed that he doesn't trust you and that really upsets you since you should be friends.
if u say it in the right fashion, not only you are going to have a new friend, but also you are going to look really alpha in his eyes