[21M] my girlfriend [19F] wants to be a sexy cat girl for Halloween. I don’t want her to

We’ve been together 3 months. She’s talked about this before but I thought she was joking because it’s kind of out of character. Then we were at Spirit Halloween with her friend and they start talking about what sexy outfits they want to wear together. All I told her was “I don’t want you to” because I didn’t want to be pushy but she pretty much completely ignored me. I was really upset because we were supposed to coordinate our costumes together and now she wants to be sexy with her friends. I decide not to say anything further, she knows how I feel about it, what she does is up to her. But I am very visibly upset and I went off on my own while they were doing all that.

She saw my reaction and tried to joke around with me but I wasn’t having it. She said we can still do our costumes together and this would just be what she wears when she’s out with her friends at a bar or party. The costume she ended up choosing was more tame than the others but that still didn’t give me much comfort.

She ends up not getting the costume because it was $40 bucks and maybe in part my disapproval. But she still wants to do it and we can’t see eye to eye. I’m trying to understand her perspective but she was mad and wouldn’t talk to me the rest of the night.

I just don’t understand why she would want to dress sexy for anyone but me. She says that’s what all her friends are dressing like when they go out but I know that’s not the only reason. It doesn’t seem right to me. I would do anything for her but wanting her not to wear a sexy costume is too much to ask.

It’s not even just about this incident. I’ve been feeling like I sacrifice a lot more for this relationship recently and when I ask for small things she blows them off and I feel unappreciated.

I don’t know. This is the first relationship either of us have been in. I’m trying my best to make it work but I need an outside perspective.

TL;DR: girlfriend wants to wear a sexy Halloween costume with her friends. I hate the idea. She doesn’t understand why I’m so upset. I don’t understand why she wants to dress sexy out in public when she has a boyfriend.

Thanks for any advice