From Christian colleges challenging ObamaCare's contraception mandate to the Catholic bishops protesting gay marriage, Christians are among the most unyielding opponents of sexual liberation. Progressives working towards greater public acceptance and support of LGBT rights would no doubt like to see Christians with traditionalist views abandon their old-fashioned doctrines on these matters. Or just go away.

Neither will happen.

These Christians are bound to their idea of human sexuality because it is fundamentally inseparable from their understanding of God. Put another way: if they're wrong about sex and gender, then they're wrong at a very basic level about God, the bible, and their religious tradition.

The problem here goes beyond the difficulty of reinterpreting a few scriptural passages about homosexuality and proper gender roles. If Christians are no longer up in arms about trimming beards or eating ham, a common progressive line goes, why must they follow the bible's similarly backwards thinking around homosexuality?

The impasse, as Matthew Schmitz notes over at First Things, stems from the fact that "sexual difference is woven into all of Scripture." According to the traditional Christian interpretation of the bible, sexual difference is ordered towards procreation and serves as the foundation for understanding God and his Kingdom, the church. "Tug on the strand of sexual difference," Schmitz writes, "and you risk unraveling the whole" of Christianity.

For Christians like Schmitz, sexual difference is more than a metaphor the biblical authors used to help their audience understand the divine. If it were only an aid to understanding, it might be replaced without harm. Rather, sexual difference is a reality God built into human nature and part of how God reveals who he is and how everyone should relate to him.

The bible presents God as Father, a name with a specific meaning. In the words of the Catholic Church, calling God Father indicates that "God is the first origin of everything and transcendent authority," attributes historically associated with the male of the species. Now and again the bible describes God in feminine language, but it never calls God by the title of Mother. According to this view, motherhood is not a sign of first origin or transcendent authority. God doesn't give birth to creation; he is separate from it. Eve was made from Adam. The wife submits to her husband. Christ called the church his bride because the church submissively receives God's revealed truth. If it sounds sexual, that's because it is.

This use of language assumes differences between masculinity and femininity, as well as a solid, hierarchical line delineating them. It also assumes true gender roles are grounded in this unchanging, ordered nature of men and women. "Everyone, man and woman, should acknowledge and accept his sexual identity," says the Catholic Church, meaning that God created each person as either male or female (Gen. 1:27) and everybody should embrace this God-given identity, whatever their own experiences and sense of self. To think otherwise "violates God's clearly articulated and intentional design for the sexes," says the socially conservative organization Focus on the Family.

Today's growing acceptance of homosexuality and self-defined gender identity challenges more than an old, persistent patriarchy. It threatens more than the Christian notion of sexual difference and complementarity. If this traditional Christian understanding of human sexuality is wrong, then the biblical authors were misguided in building their conceptions of God and the church on the foundations of sexual difference. The bible would call that building a house on the sand.

It's no coincidence that progressive Christians who support gay marriage and transgendered identity tend to reject the inerrancy of sacred scripture, at least as traditionalists understand it. With a looser interpretation of divine authorship, they're open to criticizing the bible as they would any other text. However, not every Christian is willing to give up biblical inerrancy and their traditions, believing that divine inspiration would go away with them. As long as these Christians have influence in the public sphere, proponents of LGBT rights will have to contend with them.