The modern view of sex and “gender” is decisively anti-Christian. That’s not simply because the modern view is opposed to tradition, Christian tradition like all other tradition, but because it reflects a habit of thought that trivializes Creation and makes the Incarnation impossible and senseless.
The problem is that modern sexual views deny intrinsic meaning to the physical world. In particular, they deny intrinsic meaning to the part of the physical world closest to us, our bodies, and especially to an aspect of our bodily life that, as they say, “makes the world go round.” Sex and the difference between the sexes have only the significance and function one chooses individually to give to them. And if that’s true of something as basic, as pervasive, and as symbolic as sex, what is it not true of?
But if the world is only what the individual makes of it, it’s not clear why it matters that God said it was “very good.” On the modern view, God can speak for himself. An error on something so basic has consequences everywhere. In particular, for the Incarnation to make any sense at all it must be possible for what a man says and does to express divinity sufficiently to permit recognition. The claim God was incarnate in a pebble, for example, wouldn’t make sense. But if what can be seen and heard of a man—his physical actions—have only that significance each of us gives them, how can they express anything but what we have invented ourselves? As a vehicle of revelation they become useless.
Here then is the problem: if sex has no intrinsic meaning, then neither does anything in the world. But if that’s so, then it becomes impossible for God to communicate with us through things in the world. All meaning becomes something we create for ourselves. Which is why the teaching on sex (and for that matter on women priests) is not something that can be made optional within Catholicism.