Your fantasy will probably not turn out like this.

Q: My girlfriend really wants me to let her get gang-banged by a bunch of dudes for her birthday (including me). I'm a little uncomfortable with that, but I've always wanted to have a threesome (or more) with a bunch of girls. Should I say okay on the condition that I also get to bang a bunch of girls?

A: Personally, I'd be pretty darn pissed if someone put a condition on my birthday present... "Happy Birthday! It's a puppy! But you can only have it if you buy ME one..." Sorry, but no dice. 

I don't know your life, but it sounds like this would be first time you guys would be inviting other folks into your bed. If you've been dating for a long time and don't make a habit out of such pillow parties, I would recommend strongly against letting in strange genitalia. As you said, you're already uncomfortable with the idea. How are you going to feel when you wake up in a cold sweat, having just re-lived the experience of your girlfriend getting banged by not just one other guy, but five? That shit will leave a mark on your brain.

Putting a condition on the situation opens up a lot of dangerous terrain, as well. What if your girlfriend gives you her word that you can have your own personal harem for a night, but then backs out? You'll end up resenting her. Or what if she wants to back out but feels obligated to go through with it, and ends up doing something that isn't entirely consensual. If it doesn't scar her, it will definitely scar your relationship.

It's tough to be that free with your partner, but it is possible. I once interviewed a couple who is part of the BDSM community here in Chicago, and they participate in a lot of group sex (and other things that we won't get into). In fact, the husband threw a gang-bang for his wife as a wedding gift. Their theory is if they love each other, why would they ever want to spoil each other's fun? Good point there, but STD's can definitely spoil your fun. So can PTSD from having seen your woman penetrated by someone else's penis. 

If you think your psyche is up to the challenge, by all means mess with that bull. It could end up being a lot of fun... or you could just end up getting the horns in the form of expensive therapy bills.