Thanks for sharing.
As much as I hate to make a statement like this because I agree with those who have come to the conclusion that "poly" has no single definition or model, I can only say that there's simply just nothing "poly" about this whole configuration - except possibly your own self-definition.
It's just lacking all the primary components - especially love, kindness, compassion for each other etc. It's (for them it seems) primarily about sex which is fine, but I think it's important to call a spade a spade.
Maybe this is what some others may try to point to when they try to express some negativity towards sex without (or with little or unreciprocated) loving kindness. It just is loaded with potential for bad things as you've illustrated.
Now - don't read me wrong - I have no issue with the idea of pursuing sexual fulfillment but I think it's just important to realize the potential for bad consequences unless there's more of a bond and good communications between all involved.
For example, if it were me that was going to choose to involve myself in a situation like you described, I would know full well going in that it was probably not going to last, would be walking on eggshells most of the time, and a lot of chance for drama and damage. Might I still choose to go there ? Maybe - but unlikely. The sex would have to be mind blowing and I'd have to be pretty desperate (starved) myself. But that's just me.
This type of arrangement is more typical of the type of behavior you find in swinging circles and it seems anyone who navigates it successfully went into it with an attitude of "it'll be fun while it lasts" and managed to come away with that same outlook.
Problem being - the heart doesn't care to acknowledge that attitude so you end up with a head/heart battle
But as you say - a learning experience with what appears to be minimal damage done ?