My (20m) parents (44m, 42f) went to Hawaii for a week vacation, and I watched the house and dog while they were away. My girlfriend (Lauren, 20f) stayed at the house as well most of the week. When they returned home and were unpacking, my mom became very upset about something in their room. I overheard some sort of argument, and then my dad pulled me aside and asked if Lauren and I had slept in their bed. I told the truth and said yes. I honestly hadn’t thought anything about it, didn’t think that it was weird or an issue. Their room and bathroom is the nicest in the house, and we were the only ones home. He shook his head and said “never again, your mother is quite upset.” I apologized to her that night after she had cooled down. She accepted it but said “I don’t want another woman in my marriage bed and I think you know what that means.” I feel clueless about this. Is this something I should have known was not ok to do? AITA?