As far as I can tell, the walking in on it being a turn-off and that that unfortunate experience is a continued source of turn-off (i.e., "I'll never fuck him again") is the same thing. I can understand walking in on the act while it is in progress to be a turn-off, fine. But, the guy he walked in on could have showered fourteen times, had five enemas with lavender water, and had a bidet installed in his bathroom, and the poster still wouldn't be able to forgive the experience.
So, again, look at what you're "remember[ing]" as well: the fact that this person does, in fact, need to use the bathroom once in a while. We're not talking about scat play here, just about the necessities of daily life. So, essentially, the fact that you allow the cognizance of normal human physiology, even when not being presently manifested, to interfere with your relationship with the person seems rather absurd. Wouldn't you feel somewhat unjustly slighted if somebody decided he couldn't enjoy sex with you because a few weeks ago he just happened to witness you farting or "smelled/heard some of" YOUR "massive shitting from the bathroom"? (Or are you also one of those marvelous gays with the invisible colostomy bag? Have gay people collectively forgotten that the lower GI tract evolved for the purpose of eliminating wastes, when they now misunderstand that as some kind of a sign of shame or even affront? I'd say next time you're with a guy whom you haven't had any previous knowledge of his GI activities, take a moment to reflect on the fact that even though you haven't seen it or heard it, he's sure as heck doing it nonetheless.)
I wonder, is this a hypothesis in your mind for why so many marriages become increasingly sexless over the years--statistically too great a chance to become aware of one's partner's normal, healthy bodily functions?