Insisting that there are only two sexes—male and female—is a very old belief. It can be found in Genesis 1:27, “So God created man in his own image, in the image of God he created him; male and female he created them.” Note the definiteness of human sexuality: “male and female.” This is not a spectrum like a rainbow, where the colors melt and blend from bright red to dark violet. Other aspects of our humanity may be better measured on a spectrum. Our sexuality, our gender, cannot. We are male or female. There is no third way.
Christians are not bound by what is culturally acceptable. They have a higher authority: the Word of God. You cannot follow Christ if you reject the Christ of Scripture. What’s more, the Christ of Scripture believed in the authority of the Scriptures, and he appealed to them as the foundation of his teaching, ministry, and mission.
Saying that humans are either male or female does not mean that we must conform to cultural stereotypes of masculinity or femininity. Telling a girl she shouldn't play sports because she's a girl is hogwash. Telling a boy he shouldn't learn to sew because he is a boy is sexist. Saying that humans are either male or female does, however, mean that we should embrace our maleness or femaleness in God-honoring ways.