That's a funny story, Biblio474! I have found myself in that same situation while camping or hiking with friends.
"Oops! Did I catch you at an inopportune time?" Its the embarrassment I enjoy. Their look of shame is priceless to me. The reddening of their face and the nervous giggle is what I like.
While I don't knock people who do, I don't get a sexual thrill out of seeing a female peeing. If I did, I'd just stand in the doorway whenever my wife was in the bathroom because she never closes the bathroom door to pee. I don't see that as sexual at all. Its the ENF moment that I like. I've just caught them doing something they shouldn't be doing!
