I don't want my girlfriend to go to frat parties without me
Is there good reason that you do not want her at these parties?
I'm scared that she's going to have sex with other guys
I understand your concern, but it's not right to be so controlling of your significant other. If you really trust her then you should let her do whatever.
I do trust her, but I still don't want her at that party! There will be alcohol involved and lots of other guys.
Let her go to the party then! It is not okay to try to be that controlling of your significant other.