It can be platonic, familial, romantic, anything.
I’ll start: I recently moved in with my boyfriend, who lives with his wife and their son. They were already separated when we met, but his wife doesn’t want to divorce or move out, and she wants their son to live with both of them. My boyfriend agreed to those terms.
So now I’ve found myself in a pretty unusual situation…
Porn hasn’t helped it either. It’s a popular topic but you only see the brief moments and not the after, and like you said, it takes both compatible people AND an agreement on expectations. One person strays from that, and you’ve messed things up for probably everyone involved with each other. I think it would only work if each person cared for the others at the same level, and not one being an “extra”.