Changing a persons race for the sake of it, is a dumb decision and goes both ways. Id be ultra pissed if Blade was white, or Kyle Rayner was Asian, or if Iron Fist was Puerto Rican. But we live in a PC world of comics so those changes will happen along with a transgender Supergirl
There has been a Supergirl who was a "protoplasmic matrix" and could shapeshift to look as a human woman... but somehow you draw the line at a transgender Supergirl.
Now tell me that you not only don't hate transgender people, but you even have many transgender friends, please.