My(22m) SO(22f) showered naked with her bestfriend(25F) while I was out late at a sports game am I not thinking of the situation correctly?

Am I the wrong one for being mad?? I’ve been telling her all day that it’s weird that they did that, and that I’m not feeling great that they decided randomly to do this while I was gone. All she keeps telling me is that she used to do it in highschool with her swim team. But I told her why I feel bad as they are both bisexual and that I understand nothing happened but it just makes me feel bad that they all the sudden decided to do this while I was gone and while they were both very drunk. She keeps rolling her eyes and saying it’s fine, and I will tell her why it is not. I even tried explaining if I was showering extremely intoxicated with your friend you would also be very upset at me. But when I said that she just got upset at me. She said well if you showered with your friend I wouldn’t care. I just feel weird about this whole situation, like i understand that she did thst in the past and was fine with it but we have been together for ages now and all the sudden they just decided to do that together.

EDIT: everyone, this is why I’m very confused about it. Everyone had different opinions regarding it. I understand that, everyone thinks differently, I’m not saying I hate her, or I don’t love her, I’m more of just trying to get an understanding of why she would be thinking of that all the sudden. I have gotten great responses on why, but as we see in the comments people think very differently about it. To be accepting of both sides of comments, I’m understanding of why she thought it would be ok, and I’m also understanding why it was more of a thing to talk about/feel uncomfortable about.

I didn’t make this post for people to be rude to eachother on if I am right or wrong, I more of was seeing if my understanding of the situation is correct, if what I’m feeling about it is correct(what I understand). Thank you to all those who have responded with a very 50/50 mindset of eachother!

EDIT2: since most comments are still confused, here is more information: her friend has/had herpes(this isn’t anything to do with it but it’s just for all the people who are commenting to take advantage of the situation). I will not take advantage of it, I understand others would, but I just want it be with one person. We don’t have an open relationship and she In the past even told me that people that do have open relationships are crazy.

I don’t like all the comments regarding that I’m sexualizing them together, I am not, as i have said, I am just more of feeling sad about why she would do this and why she isn’t feeling the same way as I would. I’m sorry if this sounds very “straight male” to some of you. But it’s nothing like that at all, it’s more of me feeling bad about it and her not understanding it. I’m sorry if I didn’t provide enough context I was flustered in the moment when I made the post.

The comment about my best friend is more regarding her being mad on not being invited to things even though I’ve tried to get her invited to things(he didn’t invite her to his wedding but I still went as I felt if it’s the last time I’ll see him, this will be the time). But my bff feels she’s too controlling and not changing like I change for her. That’s why he does not like her as much/didn’t invite her…

tldr: gf who I soon plan to get engaged to showered with her best friend(naked) while I was at hockey game. I now feel sad and confused and weird about the situation…

Edit3: don’t know if this updates the bots when I edit too or if it’s just posts, but we broke up lol. We went to the bar tonight with my brothers and her friend was there, when we we going home, one of my brothers who is the dd got pulled over in my car by a cop who said my cars license plate lights are out and let us go, then I got into an argument with her bestfriend cause she apparently knows the law and said putting your seatbelt on panicking in front of the cop isn’t considered a seatbelt ticket. And I yelled at her that she isn’t an attorney or a lawyer and can fuck right off with that but my yelling wasn’t ok with my SO.