Those of us who grew up with hippie parents in place like Berkeley know this territory pretty well from a kid's perspective, and I gotta say, it was always toxic and messy and narcissistic, and always involved one person (usually a masculine person), being very into it, and the others very into them.
My own experiences in my twenties did not change my mind about this. The thing about a partnership is it somewhat limits itself. There's you, and there's the other. You either work it out or you don't. When you add more people (and I have), it gets messy, it gets uneven, and there's more room for miscommunication.
And there's another issue that all of you pie-eyed well-intentioned kids don't seem to parse: the US (and many, many other countries) don't have polygamous marriages as a legal option for some very good reasons, and most of those reasons have to do with predatory men collecting child brides. Now, I'm not a stick in the mud who'd say group sex isn't fun--certainly can be. But when you factor in the potential for abuse, hey, maybe read "Banner of Heaven" again and get back to me.
Ah, but that doesn't apply to your massively enlightened and wonderful relationship? Cool. Nifty. But again, there are reasons we have protections for women in their partnerships. You're undoing them.