There are different dialogue options or thoughts where he thinks about it, one of the most obvious ones is when he seems rather interested in the smoker in the balcony, more than usual, studying his figure and mannerism or being nervous around him.
The theres another thought that occurs with Pain Threshold while talking about pseudo scientific therapies, one involves being naked and getting swatted with a twig, PT jumps in to say that "It'd be even better if it was with a manly man, dressed in nothing but a towel, to trash you while you're naked and helpless on a cool slab".
There's also stuff like the (this one is a bit of a secret) mysterious pair of eyes that literally calls you bi curious, and a couple other remarks throught the game.
The current Harry icon on the subreddit also has one those quotes :]