"Before addressing the morality of "sex change" operations, or what is more formally termed "sexual reassignment," we need to first call to mind the fundamental moral foundation governing this issue. Each person is a precious human being made in God's image and likeness with both a body and a soul. Vatican II's "Pastoral Constitution on the Church in the Modern World" asserted, "Man, though made of body and soul, is a unity. Through his very bodily condition he sums up in himself the elements of the material world. Through him they are thus brought to their highest perfection and can raise their voice in praise freely given to the Creator. For this reason man may not despise his bodily life. Rather he is obliged to regard his body as good and to hold it in honor since God has created it and will raise it up on the last day" (No. 14). St. Paul also reminds us that our bodies are temples of the Holy Spirit (1 Cor 6:19), and therefore we should not degrade our bodily dignity by allowing the body to participate in the act of sin. Moreover, such sin hurts the body of the Church. For this reason, the Church teaches, "Except when performed for strictly therapeutic medical reason, directly intended amputations, mutilations, and sterilizations performed on innocent persons are against the moral law" (Catechism, No. 2297) ... To destroy organs purposefully that are healthy and functioning, and to try to create imitation organs which will never have the genuineness and functioning of authentic organs is gross and lacks charity. Such surgery which purposefully destroys the bodily integrity of the person must be condemned."
This extract is from an article, 'Straight Answers: The Morality of 'Sex Change' Operations', in The Catholic Herald (A US publication; the link comes via the Centre for Law and Genetics).
A question the article raises - other than the questions related to the normative schema it rests on - is whether there is still a discussion to be had over sex change operations, or whether sex change operations are ethically permissible.