Why stop at double?
If three or four people want to raise a family together,* the kids would be probably be even better off than a double parenting. There's probably a point of diminishing returns--I wouldn't want to be raised by a hundred parents. But I doubt two is the optimal number.
From the parents perspective, most people aren't interested in a poly relationship. And most poly relationships aren't stable for the 20 years it takes to create an adult. So while a poly relationship might be better for the kids, it isn't always feasible.
Now I wouldn't begrudge anyone, even a ...
Recently, LWers Will Ryan and Divia Melwani (now Will and Divia Eden) were married, with Eliezer Yudkowsky officiating.
I've been to 40+ weddings in my lifetime, and this was my favorite ceremony yet. Here is the video, and below is the transcript of Eliezer's... what's it called? "Blessing"?