My Girlfriend[28F] spent two nights at a guy friend's[28M] place

Me (29M) and my girlfriend (28F) were together for 3 years. My girlfriend recently went on a one week trip to SF and stayed at a guy friend's place. This friend has been close to her for 10 years, and I've met him a couple of times before. Before the trip, she told me she would be staying there alone since he was traveling elsewhere during that time. I trusted her and didn’t have an issue with it.

However, on Friday night, he unexpectedly came back from his trip. She ended up staying at his house with him—just the two of them—on Friday and Saturday night.

I didn’t find out about this until a few days later when she mentioned it. That’s when I learned what had happened. I asked her why she didn’t tell me earlier, and she explained that she didn’t mention it beforehand because she thought I wouldn’t be okay with her staying there with him.

I told her that I wasn’t comfortable with this and hoped it wouldn’t happen again. I tried to explain that while I don’t mind her hanging out, eating meals, or drinking with guy friends, I feel uncomfortable with the idea of them sleeping under the same roof. She responded by saying my boundaries were "wrongly set" and that I’m insecure because he’s her long-time friend. She also explained that she grew up in an environment where it’s normal to have guy friends.

Now, I’m conflicted. I feel like she disregarded my feelings and boundaries. On the other hand, she says she doesn’t see anything wrong with her actions because her environment growing up made her think this is normal. How do I handle feeling uncomfortable while respecting her friendships?

TLDR: I’m struggling to handle my feelings after my girlfriend stayed overnight with her guy friend. She thinks it’s normal and I’m being insecure.