Transphobia by definition is not civilized and the rules of civilized
discourse do not apply when dealing with individuals who are external to
your civilization.
That to me seems like an absolutely extraordinary proposition. The epitome of civilization is to apply the rules of civility also to persons that you disagree with. That's the purpose of civilization. We don't need much such rules to interact with persons that we agree with. Redefining some groups of people as being outside our own civilization and therefore that our rules of civilization don't apply to interactions with them has been done many times in the dark history of humankind. It seems to often end with various forms of oppression, like colonization, slavery, or genocide.
More close at hand, if the other person, in their mind, thinks that "trans is by definition not civilized" (which is an opinion I would disagree with, just to be clear), then this argument gives them free reign to not behave in a civilized way towards that group.
/Oskar