I've never seen anyone fucking but I've heard several people.
Several years ago there was a couple living next door, she was a big woman but attractive (BBW is the term I think?
). Anyway, she used to really go for it, very vocal and no concern over volume. We got quite friendly so it was hot talking to her during the day and then hearing her being fucked hard later on - I'd often have a really good wank to the soundtrack of her fucking.
I've heard other neighbours fucking too, also, when camping I've heard friends fucking, that's hot.