AITAH for telling my sister she has no self-respect after I accidentally walked in on her boyfriend peeing on her face?

Not a combination of words any older brother would wish to write. I'm 20, my sister and her boyfriend are both 18. They've been going out about 18 months, but they've become a lot more serious over the past six months or so. He spends a lot more time at our house, including most weekends.

He gets along with me very well; in fact, he got me into football and we go play once a week. Our parents like him very much, my mother talks about how respectful and well-mannered he is. This is despite the fact that we're in the US and he's Salvadoran (I mention this because some people would consider my parents' views on immigration quite hardline, but they've got no problems with their daughter dating him).

He was staying over at our house on Friday. My sister and him were watching a movie on TV and I went to bed early as I wasn't feeling well. A few hours later, (I'm not sure what time), I got up to go to the bathroom. We only have one in the house, and we usually knock, but I completely forgot to in my grogginess.

As I opened the door, I saw what I've only ever seen in low-grade pornography before. They were in the shower cubicle with my sister on her knees, while her boyfriend was letting out a stream of urine directly onto her face. They both looked over at me and saw that I'd seen as I shut the door quickly, feeling worse than I ever have in my life.

I couldn't sleep that night and her boyfriend left early morning, neither of us looking the other in the eyes. When he left, my sister tried to break the ice but I shut her down by saying, "you need to have some self-respect rather than thinking of yourself as some cheap, trashy girl" and walked off.

This is got my sister furious at me as she told me I had no right to judge her, and that I was a creep for even bringing up her sexual life with her. I mean, how do you watch your sister getting peed in the face by a boy and then pretend you didn't see it? AITAH for reacting this way?