“A woman must not wear men’s clothing, nor a man wear women’s clothing, for the Lord your God detests anyone who does this.”
“Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God.”
“Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry. On account of these the wrath of God is coming … Do not lie to one another …”
“The one who conquers will have this heritage, and I will be his God and he will be my son. But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.”
Sex is offered, in the bible, to those who enter into the loving, lifelong covenant of (heterosexual) marriage, and no-one else. God commands us not to be sexual in any other context – not before we are married, or with people we couldn’t or shouldn’t marry, or with other people’s marriage partners, or prostitutes, or people of the same sex, or even animals. It does us great harm to ignore those rules; it makes it harder for us to understand relationship as God intended, and therefore much harder for us to relate to him rightly. Even if we’re Christians, and we chose to ignore those rules, God warns us – our relationship with him will, eventually, be so broken there will be no way back. We will not want to be with him forever, and we won’t.
But the same warnings apply to lying and deceit, and especially when we deny God-given reality about ourselves and who God made us to be. Deuteronomy 22.5 spells this out; God gave you the body you have, and you are to live in accordance with that, recognizing that God made your given biological sex and your gender to be wholly united, not divided. Don’t misrepresent or play with that. We aren’t to mess with our bodies in any way.
Why? Because that’s trying to play God. He decided who your parents would be, when you would be born, and what body you’d have. To reject those things is to reject God’s provision for you. It’s rebellion. To die your hair or pierce your ears is no big deal; to pretend to be a different sex than you are is, in God’s eyes, a very big deal. To actually mutilate working organs in the body God gave you because you think he should have given you a different body is an appalling act of rebellion.