> I thought sex != gender

Correct.

> sex field which means the biological sex

The idea that there's a binary "biological sex" is also wrong. There are many choices for what you can mean by that and for every one there will be outliers that classify as both/neither. (the points bounce up and down, but - feel free to post the specific criteria that you think avoid this issue instead)

If it's "nearest biological sex at birth, or best guess made at that time (even if in error)" then that may have little to do with identifying a human thirty years later.

Which, as others have pointed out, makes it a field of dubious utility on a document intended for unique identification.

It's about a passport field than can represent anything the issuing country wishes. So for example a traveller that has the value of this field changed according to a German law [0] to anything different than M or F (most probably other values will be represented with an X) will be denied entry to the US.

https://de.wikipedia.org/wiki/Gesetz_%C3%BCber_die_Selbstbes...

Trans people change their biological sex by transitioning.

Well, biological sex is not well-defined. See Ullrich Turner syndrome and similar genetic differences. It does not affect that many but it still affects them. Technically with X0 they would be female (since they have no Y-chromosome) but are they really female? Same rationale for other such genetic conditions.

This here has more to do with Trump pushing down a certain agenda. They also only focus on what they conveniently look as a "problem" while ignoring every other situation, such as genetic conditions.