Good question, and I think it applies to me as well, in some way.... My wife and I were married 2 1/2 years ago, when she (mtf) was legally male and I (ftm) was legally female. She has since legally changed her name, though not her gender... but in Arkansas, where she was born, she does in fact have the option of changing her gender on her birth certificate, and as far as I'm aware, that's what she intends to do.
I, on the other hand, will never be considered legally male... not unless the laws change. I was born in Ohio, one of only 3 or 4 states in the U.S. that will not change gender on birth certificates. Suppose I can only hope that when and if my wife changes hers, the wonderful US of A doesn't nullify our nuptials.
SD