Breasts aren't sexual organs, they're a secondary sex characteristic which are more developed in females than in males. Some people view them as being a simple part of the chest, and don't have a problem with going topless. It seems like your girlfriend feels this way.

That said, I can definitely understand why you'd be uncomfortable with this, as breasts are an overwhelmingly culturally sexualized part of a woman's body. You've said that you view this as your girlfriend "showing off" her breasts to her male friends. Did she say that she wouldn't mind showing them off, or that she wouldn't mind just being topless/nude, because in my head, showing off would mean inviting someone to look at them verbally, as in "Oh, hey, look at my boobs" and not just sitting in a sauna where people are generally nude.

I'm also having a little trouble understanding if this is something that has already happened, is going to happen, or is just a hypothetical situation. Would you mind clarifying?

In any of those cases, you've only been together two months, so this relationship is still very new. I would have a specific discussion about this if it wasn't a hypothetical situation, and see if you can reach a solution that would make both of you comfortable through compromise. Maybe she decides she'll wear a towel in the sauna with her male friends as a compromise. Communication is going to be key here, I think.