[MF] My [M20] girlfriend’s [F18] tits are always visible

I don’t know what to think about it, there are moments that I get angry about it and others that it provokes me. Basically, my girlfriend [F18] of two years of relationship always wears clothes that leaves her boobs visible. Whenever she wears a bra, it is to big and doesn’t fit her well so you can easily see her nipples when you look down at her cleavage. But, most of the times, she doesn’t wear a bra at all and pretty often she wears loose shirts or dresses that also allows you to get a clear view of her nipples and tits. There was one day that she even let her nip slip during a live she was doing in a social media with lots of people seeing it. I’ve already told her that most of your friends and even strangers could have already seen her tits, but she just doesn’t care. And I say that because I’ve seen some of our friends really staring it and she even recognized it, but she said ‘nevermind, let them see it, it will not kill me or you’. And I don’t have the balls to really confront her in order to give her an ultimatum so I just accept how it happens…