Well, that certainly is a pickle, Miguel. Let’s take a look at your options here…
1) give the bucks party a miss and possibly put your mate’s nose out of joint
2) attend but make an excuse to leave before the stripper arrives, subjecting yourself to a lonely, six-hour drive home
3) go away for the weekend regardless and risk the wrath of your woman
It’s delicate situation and none of the above options are especially palatable, particularly as it is you who will bear the repercussion of whatever course of action you take. You'll either have to choose between, and potentially upset, your friend or your love or severly inconvenience youself to appease them both. And all for something that you don’t really want anything to do with anyway.
Perhaps in this situation the mate might be more forgiving than the girlfriend or vice versa and so you could choose the path of least resistance.
However, there is another option: try talking to your girlfriend again. Make your position on the whole skin show clear and reiterate that it is only one small part of an entire weekend—those girls get paid by the hour so they won't be sticking around for breakfast. The thing is, you’re put in a tight situation out of consideration for her feelings. Drill down exactly what her concern is: cheating, looking at another woman or something else?
You’re clearly not gagging to see a chick in a g-string contort herself into a pretzel, so if need be try offering a compromise: you’ll make yourself scarce when the erotic entertainment arrives. Who knows, there'll probably be at least one other guy there who'd rather join you on balcony for some beers.
If that’s not enough and she objects on the grounds that she has no way of knowing that you’ll do as you say, then there is a bigger issue at stake here. It’s called trust and no relationship is worth squat without it.