The most boring, uncontroversial leftist formula is "commodification of X is generally bad" (X being some important social or cultural relation or institution, or whatever) but for some reason, lib brains get so triggered when X = "sexual relations". It's like their CIA programming kicks in and they become physically unable to think straight and start arguing against some random ass point that nobody made.
They're not saying sex work is immoral. Normal sex, as in, the work that goes into the physical reproduction of human bodies, is sex work. Your brain is so full of capitalist ideology that you think that work is only work if it's mediated by capital.