Every cat must get a mate.

* Definitely holds up. Sure, there can be exceptions where cats must give kits to their dying Clan, which can interesting! However, this is more about "every cat must fall in love and have babies." Some people just aren't interested in romance for myriads of reasons. And believing every cat must have a mate can lead to icky situations where the author must find two single cats. Totally unrelated, but Pinestar was a leader when Leopardfoot was an apprentice which is certainly... a relationship.