Does anyone honestly believe someone would CHOOSE to be transgender (or homosexual, bisexual, etc.)? Why on earth would anyone choose an extra hardship? Why would they choose to be discriminated against? Sexuality is not black and white, and sexual identity and attractions are not simply controlled by one's will.
As long as you're in a state with non-discrimination protections, this isn't much of an argument anymore. Outside of conservative church communities, society is pretty accepting of LGBTs, or at least has a "meh, I don't like it, but it's not worth fighting over" attitude.
