I will attempt to give a rational answer using the rational Bible.
I believe that Romans 1:24-28 would provide us with principles governing transsexuality.
Rom 1:24 Wherefore God gave them up in the lusts of their hearts unto uncleanness, that their bodies should be dishonored among themselves:
Rom 1:25 for that they exchanged the truth of God for a lie, and worshipped and served the creature rather than the Creator, who is blessed for ever. Amen.
Rom 1:26 For this cause God gave them up unto vile passions: for their women changed the natural use into that which is against nature:
Rom 1:27 and likewise also the men, leaving the natural use of the woman, burned in their lust one toward another, men with men working unseemliness, and receiving in themselves that recompense of their error which was due.
Rom 1:28 And even as they refused to have God in their knowledge, God gave them up unto a reprobate mind, to do those things which are not fitting; ASV
Verse 24 speaks of not dishonoring one's body. The Greek word for dishonor means to render infamous, that is, (by implication) contempt or maltreat: - despise, dishonour, suffer shame, entreat shamefully.
Altering ones gender would seem to me to be dishonoring one's body.
Additionally, if someone actually altered their gender and then engaged in sexual relations with someone of their new gender, then that would be homosexuality. And you admitted such as being wrong.