Struggling with a value difference about my girlfriend’s past

My girlfriend and I have been together about 10 months. Recently we were watching a movie where a woman kissed a man who had a girlfriend. I said I personally wouldn’t do that, even if I had feelings, because I don’t think it’s right to get involved with someone who is already in a relationship.

She told me that in the past she actually did kiss someone who had a girlfriend while she herself was single. She was honest about it and said I don’t know the full context, and also that today she wouldn’t support a friend doing something like that.

I don’t think she’s a bad person and I still respect her a lot, but it surprised me because it doesn’t match how I would handle that situation morally. Now I sometimes catch myself thinking “how could someone I respect do that?” even though I know it was before our relationship.

Has anyone else dealt with a value difference like this with their partner? How do you process it without overthinking their past?