Since Eridian’s mate for life would grockdrian be Erid’s first throuple. Rocky and Adrian look at their pathetic alien and what’s that about loving to the point of invention, Rocky and Adrian have five hands each, Grace has two holes, etc etc
"Eridians mate for life" doesn't actually tell you how many Eridians are doing the mating, it just tells you for how long they do it.
Throuples or other poly groups could be the standard, for all that tells us. Andy Weir says "Eridians mate for life" but then has Rocky deadass state that it's entirely possible that Adrian has a new mate because he's been gone so long. So there's some level of astrisk attached to "for life," even if it's legally/culturally making an exception for a "back from the dead" Dear John situation.
"Mating for life" also doesn't tell you anything about the nature of those relationships -- are you just legally married for life, or do you hang out all the time forever? Do they consider "mating for life" to be the equivalent of what we call "co-parenting"? Where you're always going to be entwined because of this clutch you have in common, but really you only see that mate when something significant going on with the shared kiddos -- and your new/secondary partner is who you are doing the day-to-day tasks and emotional bonding with? Or do they have arranged marriages for societal reasons and then mates you choose for love?
"Eridians mate for life" really doesn't tell us any of that, we just read "two-person marriage that never ends" into it as an assumption because two-person marriages are our culturally Christian, capitalist ideal. Shoot, even human cultures have had and still have multiple-party marriages, though admittedly it is usually multiple women and one dude more than ethical polyamory. But two-person-marriage-forever isn't even a universal practice for humans, why would it be for a monosexed alien that lays eggs?
If the environment is harsh enough that for most of their history Eridians died lots, they could have selected against pair-bonding in favor of relationship permutations that leave more hands available for child-rearing if one of you die. They could have selected against pair-bonding as fiercely as patriarchal inheritance customs motivated us to double/triple/quadruple down on it (in the most oppressive ways possible, for most of history).
