I think the primary driver is that it is “unnatural.” Accepting gay people for who they are is one thing. Accepting trans people for who they think they are is another. Other older cultures such as India have found a different way of accepting a third gender. Plus, much of our psychology is misandry, so its not hard to see how we have some support in that community to turn men into women physically. We have not come to terms with the misandry in our industries/institutions because devaluing men is a huge part of the “patriarchy.” Like abortion and existential feminism, these are all problems that are fueled by deeper issues that we don’t want to talk about yet. Most aren’t ready for the ontological shock.