My partner and I just had a talk about it. Basically, she celebrated her birthday today. I was on her party, and it was fun, but I left after around 2 hours to get home and relax a bit. After I arrived, a friend of mine texted me and asked me if I wanted to go to a lake and see the sunset. I agreed, we went to the lake and went swimming in it; it was really nice. Later, after arriving at my partners, she talked with me that it hurts her that I went out with someone else on her birthday, doing a romantically coded activity.

To be honest, I realize that I don't have a single clue what is coded as a romantically coded activity. For me, this was something completely okay and appropriate, because it is for me clearly a friend-thing; but my partner explained to me that the combination of going out with another person on her birthday and going to a sea, which is a secluded place, just heavily connotates it in a romantic way.

I understand that what I've done here wasn't right, and that I have responsibility here. Even though I didn't want to hurt my partner, it is still my responsibility to inform myself here on romantically conmotated things you shouldn't do in a partnership. So, dear people of Lemmy, what does constitute a romantic moment?

Edit: I've left out some information which seems to be important for the whole picture . I've copied it out of my comment and adding it here:

Me and my gf got together in August of last year, so basically 8 months ago; we were friends for half a year before that. She got cheated on in her long distance relationship before.

The friend who invited me to the sea I actually know for almost as long as my partner, from the beginning of Uni. She had a breakup from a three-year old relationship a few months ago, and I was there to support her. I didn't clarify before though if she was okay with me cuddling with people or not; I assumed it was with her, because it was okay in her LDR before - which was wrong of me. I overstepped the boundaries of my partner here.

The friend in question kissed me at the neck while I was at hers. I talked with her about it and let her know that I wasn't okay with it, to which she reacted quite hurt. She then told me that we shouldn't be friends, but two weeks ago she collapsed at Uni and I brought her home. Now we are meeting again.

While I'm writing this down, I'm actually starting to notice that there are a lot of other factors playing in why my partner is upset here. She has been cheated on in the past, which definitely leads her to feel uncomfortable about my actions, even though I obviously don't want to cheat. I broke a societally unwritten rule of not meeting people in romantically coded settings on your partners birthday. And I overstepped the boundary of my partner before by cuddling with the friend without my partners consent.