It is not "pointless nitpicking". It is very important holding fast against allowing very determined forces of hate any foothold whatever.

I argue 3 things:

1. Irrespective the truth value of your claim you should not forward that position as there are forces of oppression who will latch onto any conceit of inherent differences between cis and trans people and claim that is the oh-so-important difference around which they claim the need to organize the oppression they are rabidly looking for excuses for
2. Your claim is not true under the commonly understood nor scientific consensus of what "sex" is
3. Even under your claim—which I consider simplistic and reductive—that sex is mere reproductive capability via sperm or eggs your assertion remains false

although it can be lost

No one "in casual conversation" considers someone "sexless" when they lose their gonads to cancer, nor do you know the "sex" of anyone to whose sex you have referred in going on high-90s percent of cases by your ridiculously narrow definition—I can't imagine in those cases where you find yourself considering using either term you jam the person with a needle or jerk them off into a cup and bust out a microscope to check motility.

Finally I'm not sure what you hope to gain by your pedantry—they're never gonna let you into the car.