Dustfeather
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 