He never dumped her, they weren't in a relationship it was a one night stand, he found out from a mutual friend that she is trans.
Transsexuals do not change their sex because they are afraid of being seen as gay.
I agree if you are attracted to the person, what harm is done? Some people are not comfortable enough with themselves, they are afraid of being homosexual I think that is the issue, the "but I am not gay!!!" issue.