Girl friends..

So how does everyone feel about girl friends of your SO? My boyfriend has a few girl best  friends he's known before me and idc if they talk or hang out. He also has a friend who isn't nearly as close that is a girl that he's had sex with. She's real nice and dating a guy friend of mine now so again, doesn't bother me. But I do think I'd get jealous or paranoid if he met a new girl that I didn't know and became close to her. I think it just depends when it comes to opposite sex friends. However, my best friend won't even let her boyfriend have close girl friends. A lot of people are like that, not just her. So I'm wondering.. Why? How do the rest of y'all handle girl friends? Do you try being their friend as well or do they make you jealous/paranoid?