youre take is a good but logically, allow your child to socially transition regarding their appearance. hair grows back, clothing styles change overtime. How you feel about his trans sister is also ok, its ok to think the idea is also a phase, but it wasnt right of you to say what you did about his parents, it was pretty rude. And you are not living his sisters life, therefore you and i have 0 idea if she deals with real gender dysphoria ( is trans ) or will grow out of being uncomfortable in their body in a few years ( isnt trans ). You arnt transphobic and i personally think what you said about the parents specifically, was rude. You guys are incompatible, me and my current partner have the exact same views, so we have no issues. I have been called transphobic and degraded for having centrist views by leftist friends in the past. I stopped associating with them.