I (18F) have been having sexual fantasies about one of my friends. I feel really ashamed about it.

Ok so I’ve been having a fantasy about sucking off one of my friends (18m). The thing is, he’s in a loving relationship. I definitely don’t want to do anything about it because I don’t like the idea of making advances on someone who’s in a relationship. I’ve been told that what I’m fantasizing about is natural, but I still feel ashamed. Does anyone have any tips to help me feel less ashamed about it?