I have a US passport with an F on the sex line, and I’ve never had any other. I’m still scared of crossing the border twice. Going out would be fine, but coming back in would get it revoked, I think.
Because I’m trans and intersex and don’t pass for either binary sex.
This runs afoul of federal regulations saying that everyone must be M or F. And while my documentation all says F, I don’t actually look the part to a random border guard.