TSism doesn't seem to be specifically covered, so when I'm condemned by my baptist friend it's usually for less-specific things like "God doesn't make mistakes" and "defiling the body gave you."
Another christian explained to me that God has defined some fairly specific roles for men and women, roles for which it is sinful to cross. And since they don't believe our gender identity is seperate from our physical sex, if we transition and begin functioning as women, to them we're still men now PRETENDING to be women, sinfully adopting women's roles.
I also hear "you just know when something is a sin, God is trying to tell you it is - why else would you feel so ashamed about it, why did you feel a need to hide this?"
Kate