But the more I think about it, the idea of cats having to choose between cats in StarClan is so ridiculous… couldn't they all just be mates together and be one big happy family and love one another the same, or is polygamy taboo up there too or something?
This is one thing I prefer about the first series to the later books. Early in the series, you got a lot more cats who just had kittens as part of the whole “cats surviving in the wild” thing; there were romantic subplots, but on the whole, cats having mates wasn't romanticised too much. An example would be Whitestorm, who has various litters of kittens with different she-cats, and is described as “proud” but doesn't get into this whole monogamous humanised relationship stuff that seems incredibly prominent in the later series.
Something that's never come up in the series is a litter with more than one father, which is a fairly frequent occurrence in real life; as interesting as that would be, however, I just don't think the Erins would include it given the direction they've taken the series in
Admittedly as a polyamorous person myself I'd be very happy if there were canon poly relationships (especially since, as I've described, cats aren't generally monogamous anyway, so it'd be more natural) but the chances of that happening in any work of fiction, let alone Warrior Cats, is sadly incredibly small
That's actually one of the things I liked about the first series too – the realism when it came to romance, at how fathers were never really around too much but then it was still adorable when you'd get little mentions like Whitestorm being proud.
I feel like in warriors, poly relationships would be realistic when it came to StarClan. You always have those fans who go “Who will Firestar choose?” (which, again, is extremely stupid) “Who will Graystripe choose?” “Who will Crowfeather choose?” (when he deserves none of them), etc etc etc. but StarClan is supposed to be a peaceful place. If those cats still be having issues and fighting over mates all the way into the afterlife, then I'm sorta worries about how the living cats look up to them as being so wise.