Since human nature exists, sex does not amount to either biological or chemical properties in us; nor does it amount to a fluid identification of gender.
Human nature encodes capacity and purpose into humanity. Biology and chemistry can describe our constituent parts but insufficiently do so. We need to also define our capacity and purpose.
This means somewhat controversially that whether we have an xy, xx, or xxy chromosome pattern, these biological and chemical compositions do not tell the whole story.
XXY and Sex
In particular, someone born with an xxy chromosomal pattern does not fall into standard biological paradigms of sex in which male is xy while a female is xx. Were biology/chemistry alone the answer, then we’d have a subset of humanity unable to truly fulfill their sexual capacity (xxy).
It is a trap for Christians especially, then, to fall into the notion that biology and chemistry simply define sex contra notions of constructed gender. It is also incorrect to claim biology has no role in sexual determination and so concentrate only on constructed gender as determinative for sexual expression.
Biological sex still plays an important if insufficient role for answering the questions of meaning and purpose behind sex.
What Is Sex?
The answer then involves two sets of explanations for the same reality. First, it centres on what the Bible says about the purpose of sex (Gen 1–3, etc.). Second, it considers what nature says about the purpose of sex. Both Scripture and nature agree since they refer to one and the same reality. But revealed truth goes further and adds significant spiritual clarity (e.g., Eph 5)
I cannot make the argument here (but have elsewhere) that the essence of sex is: the capacity for fatherhood and motherhood. For a published argument, consider J. Budziszewski’s The Meaning of Sex.
And once these questions are answered, they give freedom to someone who does not exhibit regular chromosomal patterns. The end of sex is union with God in Christ (Eph 5:32). Sexual capacity no matter our chromosomal makeup points in this direction.
No matter our genetics, we all have one human nature. And Christ united to that nature through his hypostatic union. And by the Spirit, Christians with one human nature (the same as Christ’s) unite to God.