LOL at this. It's all perspective........and how much booze was involved.
Listen, There's no way we, who don't know her or you can judge how you should react! Personally, I don't see the big deal. If she is showing off and she's hot, relax she's with you right? Be proud of the fact that she's with you. Or were you embarrassed? She's not giving hummers for beads right? She likes the attention, and it's no different than the guy who rips his shirt off in the club to show off. Why did the guys go a separate direction from the girls? If it's all good it should be all good in front of each other. That's the part I don't get. Different personalities will be more accepting of things like this. I've dated women who went topless at the beach or lake that weren't strippers. I dated a stripper who's top stayed on unless she was working the stage. My wife is ok with topless as long as she's not the only one with 'em hanging out.
If you are confident in your relationship you would be comfortable with talking to her about it. You should have told her it bothered you. Maybe she's showing you how you make her feel. Who knows? Either way you have self confidence issues, and it sounds like to me like she had no clue your boundaries are different than hers. How long have you been together?