Well. I've been with more people at the same time.
I am female, have been with a woman at the time I was with my long-term bf, he knew about it. The gf went, after a while I lost interest. My bf also, had another gf a while later. We went to a recital of hers together. They parted after a while, when he noticed she wasn't as "interesting" as me (his words, not mine). Eventually, he broke up with me because he wanted another, simple, painful, but true.
This is, for me, the truth when it comes to polyamory. You CAN love multiple people at once, but you'll always love one more. That is my experience, and I would never try this when having kids. It's not fair to them, when you yourself are having this experiment, fine. But with kids, no. They do not deserve this confusion, it's never easy, trust me. It's worth a try, but when having kids, you should have worked this out.