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.
Or you can make plans with your friend on another day. it is generally considered the "socially acceptable" thing to prioritize someone on special days like birthdays. Even if you have only give a bit more priority on someone's birthday, I would think that a partner already has enough extra priority that "I will spend time with only you" is not an unreasonable request. It's also not clear how long OP has been in this relationship. Based on the miscommunications, probably not long. If it's a serious relationship, then giving your partner extra priority sends the message that you find the relationship serious. Otherwise the message is "you are a priority person in my life and this is a priority day for you, but this other person is even higher priority still."
If you ask someone how they feel, and express they would be hurt, but you do the thing anyway, then you are saying that you don't care if you hurt them, and "Well i didnt know it would hurt you" is now a lie. Your options are to either not hurt them (by talking through the situation until it doesn't hurt them, or simply not doing the thing), or hurt them. But if you hurt them willingly, you are the one doing damage to your relationship. If you don't think they are being reasonable, then you may be in a toxic relationship and should end it. If you just don't care about their feelings, then you are definitely in a bad relationship and should end it. Asking isn't about "permission", it is about communicating that you value their input and their feelings.
Life is complicated, so "priority" doesn't mean that something is the only thing that matters but it does mean that it should factor into your decision making.