Short answer: a small number of joyless people who have self-appointed as gatekeepers on the mainstream dating sites who religiously prowl for any accounts that step outside the usual relationship dynamic.
I'm not much for SB arrangements (but, no harm, no foul), but man, the hoops to jump thru to finding consenting adults who enjoy joining couples (the mythical 'unicorn'

) is way more complicated than it needs to be. As much as I enjoy the alternative dating sites (fetlife, etc.), they just don't have the reach of a Tinder or Hinge.
Wish more folks took the live and let live approach!