"Christ is our Lord and master, and we live by his rules and under his authority, not our own."

While reading an article in Christianity Today on "What would a Christian conversation about transgender look like", the following stood out, 

"John Wyatt, a Professor of Ethics at University College London, describes the Christian view of self as an 'art restoration view' – we are masterpieces made in the image of God, yet battered and marred by the primeval fall of humanity. As redeemed Christians, we are in the process of being re-made and restored. We were not what we were, and yet not what we shall be.
This means, then, that our personal experience, although relevant, cannot be accorded a decisive role in discussions about this or any other ethical issue. All our experience is marred by human sin in some way, shape or form. This should lead to a certain humility in discussion, and in the presentation of our own experience as part of it."

Ah, someone who might agree with my proposal for Humility Month.

"All the differentials that should mark out a Christian discussion about transgender are, of course, products of a completely different worldview to that of our prevailing western culture. The shocking EllaOne advert for a 'morning after' tablet now on UK television exemplifies a non-Christian view of ethics: 'This body is mine,' the narrator says. 'What I'm doing right now is the right thing for me, because it's my future: I am my own master. It's my life. My rules.'"

There is the problem of autonomy again.

"But for Christians, of course, it's completely different: 'Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honour God with your bodies,' (1 Corinthians 6v19-20).
That's because we are not our own master, and life is not lived by our own rules. Christ is our Lord and master, and we live by his rules and under his authority, not our own.
And, as the influential church leader John Stott said: 'Scripture is the royal sceptre by which King Jesus rules his church.'
This means that a Christian conversation about transgender is shaped first and foremost not by our own experience, or prejudice, but by the Bible."

Uh oh, I am afraid the conversation with the "transgendered" will end right there once you start mentioning the Bible.

I think you first have to understand your opponent's concepts of God and the theological worldview that they are probably unwittingly following. If their notions about God are wrong, you won't get very far until those are straightened out.