As a dramatic oversimplification: how dare you tell us that forcing domestic pets into a vegan diet is animal cruelty! We're going to make our own instance, with blackjack and hookers!
There was some stuff about vegan users being harassed after the "vegan pets" debacle too, but they're effectively "We need our own space where we can be as dramatically vegan as we want to be with no opposition".
I can see the logic. The realities of day to day life and managing a household together are a considerably larger presence than most people realize until they're shoulder deep into adult life.