Originally Posted by Himeno

It will likely depend on how far you've transitioned.

They don't care what the gender marker on any ID might be once you've past the TDC (I've been traveling with new name and corrected gender marker for a year, but they won't let me go on HRT until work and family know about the whole thing, and there has been 'complications' with that). I've had more 'issues' with the gender marker listing F but still appearing otherwise from airline staff then from security or immigration staff (mostly confusion between name on boarding pass/manifest and appearance - once they've seen passport/pass, there's no problem).

The "issue" with the mentioned case last month was that she had been taking HRT and presented at the checkpoint as female, then opted to use the body scanner. As she appeared to be female, the scanner operator selected the female option. However, since she had not had surgery, the scanner alarmed and things went down hill from there.