I agree dehumanization is bad.Misgendering is a thing even outside of trans people. Everyone has met someone who they were unsure of gender and many may have addressed a person as sir who is really a madam or vice versa, usually to the embarrassment of both parties. Or telling a parent they have a beautiful baby girl/boy when it is actually the opposite. Mostly it is innocent mistakes (where embarassment is punishment enough), sometimes it is malicious. How do we deal with the malevolent jerks? It is a form of dehumanization, not all that dissimilar to referring to an ethnic group as 'vermin'. Dehumanization is the first step to violence. Like our GOP friend in Texas who said that the only good Democrat is a dead Democrat. Whether or not it is/should be illegal, it is wrong and it is disgusting.
However it isn't dehumanizing to simply disagree with your opinion.