AITA for suggesting that my (31m) gf (27f) wear a less revealing Halloween costume?

My girlfriend and I have been dating for two years. Our Halloween plans this year were to go to a party hosted by a friend of mine where the guests were mostly be my friend group. She is a big Sabrina Carpenter fan and decided to go as “goth showgirl”, the goth part being to meet the party’s “black costumes preferred” theme. She showed me the costume the night before the party and it was just a black bra, black panties, and a sequined black see-through babydoll. She looked incredible but I was suprised by how risqué it was - that it was literally just lingerie. I told her that I personally loved it but that she might want to go with something else for the party.

I know the cliche thing is boyfriends getting mad at their girlfriends for wearing revealing clothes as some sort of possessive rage. That’s not what this is. She can wear what she wants and I prefaced what I said with that (and kept saying it, many, many times in the three hour argument that followed). I told her she can wear what she wants but I needed to express to her that this is truly not the vibe of my friend group and nobody else would be dressed even remotely like that. And she had recently expressed to me that she doesn’t feel comfortable around this friend group yet, which is totally valid as she’s only met them a few times. I just didn’t want her to feel awkward given that she already doesn’t feel accepted in the group and would be the only one there in a revealing costume (not to say these friends would make fun of her or anything - they are very welcoming and kind people). I was merely putting myself in her shoes and articulating how I would feel in this situation. Not to mention, the party was partially outdoors and it was forecast to be 40 degrees.

Well she did not take this well, accused me of slut-shaming and triggered a lonnnng argument that almost ended with me sleeping on the couch. I offered suggestions such as adding tights and shorts, which she shot down. And to be fair, she did show me the babydoll when she ordered it. I just figured it was a component of the costume and not the entirety.

She ended up going to the party in a different costume and told me after the party that I was right and she would’ve felt uncomfortable. But I still feel like I was too close to the possessive boyfriend cliche for comfort and I could’ve handled it better. So am I the asshole?