So, if you are a MTF who married a woman while you lived as a man, you can remain married to her. However, if you and her should divorce after your legal status is changed to that of a woman, you have to marry a man if you ever want to get married again.
However, the way corporations treat transgender unions is another story. Specifically, a MTF married a man after her changes, but was denied her husband's UPS medical benefits for other, non-transgender-related, procedures. A federal judge in Minneapolis ruled that she can be so denied. UPS, for its part said that the denial was due to a "clerical error."
Perhaps it wasn't a deliberate omission on UPS's part. But I can't help but to think of how many other organizations are denying benefits to the transgender partners of their workers, not to mention transgenders themselves.