They believe that God is completely fair and just, and wouldn’t make someone in a way that was contrary to how he wants people to be.

This simply can’t be supported though. Christians tend to be gender essentialists, but there are a wide range of genetic defects where people appear to conform to one gender while not having a genetic makeup that conforms to the assigned gender, e.g., androgen insensitivity syndrome. So if people are made exactly how god intends them to be, and sex is an essential part of that, how can people exist that do not genetically conform to their assigned gender, or any binary gender? You can’t say that god doesn’t make mistakes and say that sex must be binary, when genetic errors prove that at least one of those propositions must be false.