Congratulations on being invited to an orgy. That is what an "erotically themed" party is.
This is your girlfriend's way of saying she is bisexual and digs screwing other guys.
Can you handle it?
The payoff is you get to freely experiment with your own bisexuality and you get to screw other girls.
My only advice to you is wear a condom.
Do I have any experience with such parties? Well yes, but my experiences won't necessarily translate into anything meaningul for you because every scene is different. The common denominator is that everyone seems inexcusably loose before you get there and after meeting the folks they all seem so normal. In some cases men are the equals of women and everything is negotiated, in other cases men are subordinate to women (expect lots of guy guy contact) and in others (more commonly) they are male dominated (expect to see your girlfriend getting it on with other chicks and if you allow it, other guys).
Have a couple drinks, relax, stay well away from the drugs and bring a box of condoms. Regardless of whether you find this to be your thing or not you will have fun and experience something new. Just remember the cardinal rules. No more than 2 drinks, no drugs and condoms always.
If you enjoy it hang on to your woman because her breed isn't that common and if you don't get rid of her politely and move on. Chances are you won't encounter the same thing twice.
