I am not sure if this qualifies because it was leading up to a sexual encounter, not during sex.
When I was in my 20's I met a guy I was attracted to and after some good conversation I followed him back to his place. As we were undressing he had his back to me and I noticed brown stains on the back of his briefs. When he pulled them down there was shit on his ass where he had not wiped. It was such a turn off I started putting my clothes back on and left.
Aside from that I have not had a gross experience.
When I was in my 20's I met a guy I was attracted to and after some good conversation I followed him back to his place. As we were undressing he had his back to me and I noticed brown stains on the back of his briefs. When he pulled them down there was shit on his ass where he had not wiped. It was such a turn off I started putting my clothes back on and left.
Aside from that I have not had a gross experience.