I think what I am about to say applies to the ideological frameworks of most conservatives but for most it is unconscious where as for Catholics it is active and explicit. Catholicism is explicitly built on a framework of neoplatonism and in this structure there materially exists a higher plane of "ideas," which they equate with god, where the ideal form of all things exists. In this framework a trans person "changing their body" is a contravention of this ideal world as created by god. From here you get discussions of substances and appearances and characteristics, it is all EXTENSIVELY thought out and logically valid just really really stupid because the apriori assumptions are really stupid. This is one reasons that the church came out vocally against trans people as directly attacking god, because since humanity is made in gods image changing gender identities attack the idea of god. I am sure an actual neoplatonist/catholic will come along and say I am straw manning them or getting their beliefs wrong or something, but honestly this is the rough just of it as I remember from theology class in HS and the courses I took in college; and tbqh I don't care because it was never convincing even when a priest taught it and I had to memorize it for an exam.