My girlfriend was at a party and went skinny dipping. Later, a guy asked to come along and they skinny dipped with him. How should I feel about this?

First, she and the girls get drunk and skinny dip together. They go back to the party and tell their tale, and their guy friend wants in. So they go, and she tells him to "not look." I'm sorry, but as a guy, I'm pretty sure "he looked." I know this is not cheating, but it makes me uncomfortable. Is this unreasonable of me, and if not how do I bring it up with her?