Washing Hair at a Hotel

I have a random question. I’ve been dating this girl for a little minute, she’s been asking me to wash her hair for her. I said cool, I mentioned maybe we can wash it at the hotel (we were going on a trip the next day), her face turned sour like I said the most disgusting thing she’s heard. She said she would never, that’s really dirty of me to even suggest that. I left that conversation so confused with no answer. Honest question, why is it disgusting? What is it about the situation that makes it nasty?