A core tenant, perhaps the core tenant, of veganism is that animals have a moral worth comparable to that of humans and that we have similar moral responsibilities toward them that we do to other people. When a carnist tries to tell you that you're not allowed to compare animal oppression to human oppression, they are demanding that you implicitly accept the carnist view: that animals are inferior and lesser beings, and therefore we can brutalize and kill them without compunction. The comparison between animal oppression and human oppression is only degrading to humans if you start with this premise.

It has nothing to do with being considerate to marginalized people (as they claim) and everything to do with creating a left-sounding justification to make your views verboten. If you are a minority discussing your own experience as a marginalized person in relation to animal oppression, carnists will try to dictate how you're allowed to talk about your oppression and use your solidarity with animals to get you into trouble. I have experienced this firsthand. They'll happily silence you in this manner because, for them, their concern was never with marginalized people - it was with their perceived right to not only enable the endless horrors of carnism, but to never be made the slightest bit uncomfortable about them.