Sex refers to the physical and biological makeup of a person. Gender refers to the socially constructed norms associated with each sex. So in essance, sex is hard to dispute but gender is flexible.
The U.S. law has set up a dichotomy in which only two possibilities exist: male or female. But what about the 6% of the U.S. population born with Gender Dysphoria? They had their sex arbitrarily decided for them by a doctor because they obviously didn't fit in the dichotomy.
As a christian, I'm supposed to believe in this dichotomy, but clearly some people don't fit. Where do they belong? Do they belong at all? I guess my problem is...sex doesn't seem to be so rigid as we think.