“Biblical marriage is between one man and one woman.”
No it’s not.
“Biblical marriage” is between one man, one woman, and that woman's slave. (Abraham, Sarah, and Hagar)
“Biblical marriage” is between one man and two sisters after he works seven years for each of them. (Jacob, Rachel, and Leah)
“Biblical marriage” is between one man, his wives, his master’s wives given to him by God, and the wife of a man he murders. (David)
“Biblical marriage” is between one man, 700 wives, and 300 concubines. (Solomon)
“Biblical marriage” is between one man in his 20s and one female child in her mid-teens. (Joseph and Mary)
The vast majorities of marriages depicted in the Bible are between a man and multiple women OR between a man and a female child who was purchased through dowry.
So when Christians talk about marriage today, we should be focused on what is Christ-honoring, not what is “biblical.”
If your definition of a specific gender would require you to take someone into a lab for testing in order to get a result... you are lying and confused about gender.