Girlfriend hangs out with other guys, advice?

Ok I never been a jealous person but I seem to get jealous because my girlfriend haves a lot of dudes as friends. I love her so much and recently she chilled with one of them alone. I trust her and know nothing happen, however I felt uncomfortable with her chilling with other dudes alone. I just need some good rules that we both can live by when talking to the opposite gender, because I don't want to feel like I'm controlling her.
Girlfriend hangs out with other guys, advice?
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