If Nolan thinks of Marks mom as a pet, yet had sex with her and made a child with her, then what is the standard for pets on Viltram… Either theirs just a bunch of random species of people who are kept as pets, or they have normal pets(for them) and… um… do things… with. them…
Let’s just say we get to see how Viltrumites think about other species later on in the show. That said, they never have babies with any aliens that can’t communicate so it isn’t actually like a pet relationship, Omniman was just teetering on the edge of seeing his family as subhuman (subviltrum?) and that was the analogy to explain it to mark.