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.
Yesss, this so much!! Maybe it's because the first arc had so many new cats that the Erins didn't have the time to detail who exactly was mates with who, but still. It makes so much sense for wild cats to have polyamorous relationships, but I guess in the Erin's process of making cat behavior more “acceptable” for a children's series, they made them all monogamous. As the series went on and readers got to know each cat, and the Erins focused more on 'interpersonal drama' as a driving force for various plots, suddenly every cat is monogamous and there's a ton of relationship drama everywhere you look. Either way I'm pretty sick of it; I really wish they'd include polyamorous relationships in the books, but I can't imagine that happening any time soon, sadly.