The term Safe Sane & Consensual means that whatever the people in the BDSM relationship are doing, they're doing it because they ultimately want to, and have talked it over and believe there is no danger. What seems sick to one person is hot and sexy to another, everyone has limits, and these vary. Scat doesn't interest me at all, but watersports I do enjoy. I wouldn't piss on/in my slave if he found it too much, pushing his limits.
BDSM stories can be extreme, they're extreme because it's fiction, and there are no limits in fiction. Sure, forced sexual slavery and prostitution exists, and that's entirely different. I expect people who write/read such and act out such to find it a turn as a fantasy, as a roleplaying scenario, but nothing more. The way I see it, they're made up for entertainment but based on facts to varying extent. It is perfectly acceptable that you as a sub would not engage in scat and watersports, because everyone is different, so there isn't really any 'but' about it.
Welcome to the community and this forum, enjoy your stay!
I hope I make sense and am not being too all over the place with my explaining - I'm tired.