I Don't Understand

I feel like I'm falling behind everyone and not seeing things that they do.  There're people going out and making friends; some people making best friends of the opposite gender.  I don't understand this too much.  Now, I do have female aquaintances, but I don't understanf how a guy and a girl can become best friends without expecting anything more.  What if one of them get's a significant other; then there will be at least some jealousy going on.  What if one of them gives off a wrong vibe and something they're regret happens.  Especially here at Penn State, where everyone is friends with everyone and you have no idea who's with who and how serious they are.  I just don't undersand.  I guess it's because I've never had a best friend that's female or never had a girlfriend before.