It's about humiliation and degradation. I'm not saying those two things don't have a place in the bedroom. Every couple/pairing is different. I just don't understand the leap between using a person's body for ones own sexual fulfillment and turning them into a toilet. I'm trying to picture what kind of woman find herself laying on her back with her ass in the air and a goddamn bright red funnel sticking out of her while someone pees into that funnel, and what kind of thought process is involved to make her 'love it'. I think it's an easy assumption to make that whatever that thought process might be is not a healthy one.
I've had dealings with only one pro watersports guy (that I know of), and I only found out about it after the fact when he complained he'd been completely alienated by his social group because he asked a hook-up to piss on him. The guy was a fucking lunatic. Sex with him wasn't at all fulfilling or sexy because it wasn't about getting off, it was about indulging all his weird neuroses and complexes in a physical way. Maybe it's unfair to use him as anecdotal evidence that watersports can't be an healthy/normal thing, but you go by what you know.