Why is it, that girls can do one thing but when their boyfriend does the same they are an ass?

Because we don't look at it the same. If a girl hangs with someone her boyfriend hates we don't think we're bitches or anything. But when our boyfriends hang with an ex or something we get mad because we think they still like her or something.

This is kinda funny to me but makes sense you've brought it up. Jealousy is an annoying thing so don't go overboard and tell her to take a freakin' chill pill as well. Perhaps the girlfriends are hanging out with their single friends and helping them out? We've all had to help our friends out in that area.

Next time your girlfriend has an issue of you going to a party with girls there (um, that's the most immature thing you could get mad about), make sure that you calmly tell her that you wouldn't do anything more than she does with her friends when they go out. If she freaks out, then you know she's doing something wrong and a further discussion will follow. In the end, you're only being fair!

Lol bro, sounds like your girlfriends are. Aren't ideal. Gotta be careful when you generalize and say that all girls do that tho, cos I'm assuming (hoping) that the nice ones don't. But I see your point and I do believe that to some extent there are double standards when it comes to these sort of situations.
Lol I find it funny that ( if its as you say it is) that your girlfriends think that they are the only chicks in the world XD