There are probably sub-conscious patterns of antipathy to transexuality that other non-traditional sexualities don't engender, because widespread sexual counterfeiting would be disruptive to society. We have certain expectations of others that we meet, and TG violates those expectations in a domain (sex and reproduction) that triggers our most ancient and visceral feelings.

If you bit into a steak and found that it tasted like chocolate, you'd be confused and disturbed. Now multiply that feeling by a large factor.