"I've wondered if a man who was once a woman has ever married a woman who was once a man."
Ouch! It hurts trying to imagine that. Would it be a legitimate marriage?
It would have to be legitimate. If each switches sexes, the state would either recognize them as the other or not. If they recognized them as the other, it would be a man-woman marriage. If they didn't recognize them as the other, it would still be a man-woman marriage. If this hasn't happened, someone will do it for the Guiness Book of Records.