I can't explain to my girlfriend that I don't like her hanging out with other guys without sounding like a crazy jealous person. She's at the beach right now on a Monday at 10:00pm. I want to trust her but I can't stop thinking about it.
The key to a good, successful, grown up relationship is to communicate. Tell her your feelings the can either respect them or not and you can only accept her behavior of not. But you need to discuss your feelings, maturely. Not like a psycho, jealous little kid. Like an adult who has feelings and concerns.
Personally, I think it is trouble brewing. Males and females interacting will always have or develop sexual attraction to each other. It comes down to how realistic she is when it comes to the social ladder where friends and SO rank on it. For most normal people, their SO will always rank higher and will take priority over friends. But there are some that cannot live their life unless they are surrounded constantly by a dozen friends and constantly going out and spending time with them. These people will never change and will never understand what you are feeling because their life is all about their friends. You still need to talk to her and tell her how you feel, but if she doesn't see it or she will not comply then you will have to decide to either let her go and trust her, or just let her go!