(kicks down the door) I think I've got it, I think I've got how Eridians can be an intelligent species that has free will and also mates for life, and it has to do with thrumming and eggs.
So, what is a thrum? Andy never really explains it all that well beyond "Eridians can become ad hoc neurons in a group mind" but that does tell us a little bit about how it works.
An Eridian is, essentially, a colonial organism that has a shell with cells and a big honking hunk of crystal inside. The brain isn't actually organic matter, the living part of the Eridian is the colony. So if an Eridian is a bunch of cells communicating with a crystal to think, what is stopping those cells from communicating with other people's brain crystals?
I think thrumming is basically an Eridian's cells signaling to the brains of other Eridians and taking their whole colonial organism thing to macroscopic scale. A thrum is, essentially, a superorganism.
What does this have to do with mating? Well first we need to talk about eggs. Eridian eggs are a shell with cells and that presumably gets a hunk of crystal inside at some point, if it's not there from the outset. Not actually much different than an Eridian, when you get right down to it, the colony cells inside the eggs are even from the parents direct population.
Now, fun fact about eggs irl, they do have a ways to protect themselves from infection. And we know the Eridian ecoystem is chock full of microbes just waiting to pounce on an egg with a membrane permeable enough for cells to pass through it. So there has to be some way of defending the egg against pathogens, and this method is not likely to be the burying aspect of Eridian egg laying. Microbes live in dirt and all. Eridians don't have an immune system the way humans do, and they can't seal off the egg with a cuticle like chickens do, and I doubt they have many antimicrobial compounds in their system the way a frog does considering their makeup (they have plant-like and animal-like colony cells for cellular respiration, that's a lot of different microbes to not accidentally kill). So, I think the eggs do a very rudimentary version of what an adult eridian would do: heat up for a little to kill whatever's inside.
Except, one problem: how do you know not to kill off all of the other egg's worker cells? Because while Eridian eggs merge and allow the worker cells inside to compete for resources, killing off the other parent's entire line isn't great for genetic diversity, which is presumably what the goal is since Eridians don't just lay an egg and watch it grow into a little mini me.
Well, you've gotta be able to inoculate against them. And how does one do that?
What if thrumming with just one person, or even a couple people we don't know what Eridian polyamory standards are like, trains an Eridian's worker cells to recognize their mate's, so the eggs don't go into an immune response when they sense the incoming cells when they merge?
It would explain why they mate for life, being in a sort of hivemind with your partner would make your bond with them pretty strong I'd imagine, and why Rocky is concerned that Adrian has left him. Their cells no longer know each other after being apart for so long-- it's not that they don't have rock divorces, it's that divorcing an Eridian has a biological component to it as you desynchronize your cells or whatever, and being apart for so long under this system would mean essentially that Rocky and Adrian have to court and resync themselves all over again, and Rocky doesn't know if Adrian is gonna wanna do that.
Because in species that mate for life irl, there are circumstances under which a mated pair can divorce. Like most biological concepts, "mating for life" isn't actually an all or nothing situation. Poor food conditions, failed breeding, etc can all cause a divorce in a mated pair, it's just that the rate of divorce is exceptionally low in species we consider to mate for life. Even the albatrosses and swans are known to divorce.
So, I think having a biological component that would not only make the initial bonding period take longer (humans typically date for twoish years before considering marriage, and that can be shorter or longer depending on cultural or socioeconomic factors), but would also make the bond much stronger than what would exist between what we can conceptualize as a married couple, would explain the Eridians very very low divorce rate. They just don't want to divorce most of the time, but they can and will if they're not happy.
Now, is there a cultural stigma against divorce? I don't know, probably, but it also probably varies from culture to culture. Strictly biologically speaking, ↑ this is what I think is happening.
