i had a boyfriend once that got jealous of me talking to my long-time male friends, and i mean i've been friends with them for several years and never dated them. he was the only one who ever got jealous. at the very beginning i thought it was flattering but not for very long. once his jealousy got old then i began thinking that he was very insecure with himself and that was a big downer to me. we eventually ended up breaking up:(
has your boyfriend known this girl for a while? if so, then i would let him talk to her. if he wants to be with her instead of you then, quite frankly, there is nothing you can do about it. just go with the flow, be cool when he talks to other girls. he will see how cool a gf he has, your confidence with shine through because of this and he will probably end up thinking long and hard before he strays from you. guys really dig a woman with a lot of confidence in themselves.