Most Helpful Guy
If you aren't up to go in this outfit, then don't. It's that simple. You don't have to go if you are uncomfortable.
Having said that, you are going to be there with all the other girls in lingerie too. I'm sure some of them will be dressed in much less than you'll be in, and you're going to an adult party where people aren't going to be judging you negatively for what you're wearing; in fact, just the opposite. In this context, what you will be wearing is APPROPRIATE, as long as you are comfortable in it.
IMO, you should go and have fun, and not worry so much. Everyone's seen a butt before.