I am personally in favor of a longer definition along the lines of "A valid marriage requires one person to be born male and remain male throughout the marriage while the other person must be born female and remain female throughout the marriage. If either or both undergo sex reassignment during the marriage then the marriage becomes null and void."
I believe that the Constitution is a very good place for defining the basic terms that underly our form of government and the minimal definitions needed to guide the society as a whole so as to be governable through a constitution. If we are going to take the time to define marriage in the constitution, or through the less effective forum of congressional legislation, then we should take the time to make it crystal clear.
After all, the tax code requires reams and reams of paper to describe in order to close all of the loopholes average citizens might be able to take advantage of while keeping all of the lobbyists' loopholes open wide enough to fly Lear Jets through.
That just it, because the laws are so vague that intersex people can be banned from marrying anyone this is how these amendments will be attacked.
Of course, I believe the government should have NO say on marriage one way or another when dealing with adults; it opens up too many doors for abuses of powers to have it any other way.
I don't think that would work. Judi & Mikayla could meet after Mikayla changed, and they'd be to all the world a lesbian couple. But under your proposed language they'd be a legal marriage.
I think the problem is that biological reality doesn't provide the bright, easy line we wish it would. So any legal definition that tries to both enforce a bright, unambiguous line and to deny gay marriage will necessarily grind up those people who are born living too close to the line. (Whether plain gender dysphoric, or physically intersexed, or that TLA someone mentioned upstream that I'm too lazy to look up right now, etc. :-)