Hard to say, but I especially like seeing people pee in sweatpants or similar, and also more "extraodinary" stuff like wetsuits, hi-vis gear...
It's also a nice plus if something is completely staged but they try to act like it's not xD.
I think I'm not really embarrassed about it, but the reason I don't tell anyone about it is more because they'd likely find it gross and then every time they talk with me or think of me this thing comes to their mind as it's connected to me.