(US-centric)
(this is my passion and the fire that motivates me, because some governments would dare to tell me that my marriage is not as good as someone else's, so forgive me for a long and rambling post)
The answer, in nearly every situation, is that nobody knows.
There's no definitive federal case law. There's no federal definition of a man or a woman (just a bunch of administrative rules at different agencies, which don't all agree with each other). And, at the state level, there is rarely case law. The few states with case law are all over the map, and most aren't binding over the entire state.
If you cross state lines, things change. They are different at the federal level than the state level, but state interpretation of your marriage would affect how the feds see it (likely they wouldn't consider you married if the state you were in at the time didn't either, but they still might not even if the state saw you as married).
It's a mess. It's one of the reason, as a heterosexual, I'm in support of marriage equality for everyone. That's the only real fix for this.
And it's not any better where a spouse has transitioned before marriage. The only safe combination is two people who have transitioned and are opposite sex on all legal documents, or two people who haven't transitioned and are the opposite sex on all legal documents. For those of us where both partners don't have ALL the "right" chromosomes, hormones, original birth certificate identifier, external sex organs, gonads, etc, the US is a lousy place right now. The issue becomes moot once same sex couples can marry, so that's the direction the US really needs to go.
It's clear that the extremist religious groups trying to run the country don't like anyone who doesn't fit their inaccurate ideas of sex and gender (I say this as a Christian who isn't one of these extremists). And they think anyone who doesn't fit their ideas of sex and gender, or follow the "proper" gender roles (only be attracted to the opposite gender, for instance) should be punished. That is their motivation - to save all of us from hell by making our lives hell. All I can suggest is to do what *you* need to do to be happy. Don't let the religous extremists win. It's what my wife and I did. I'd rather have the government take all my assets for claiming my wife as my wife than to deny who she is or live without recognition of our love. Love is a higher law than state or federal law. And I *dare* any religious nut to tell me, to my face, that I'm wrong.
(I am not a lawyer, so take this for what it is worth - not much) Regardless, you both should have a will and make sure that things like medical power of attorney are explicit. Make sure that the person you love has the legal rights, not some random family member. You probably will never face a challenge to your marriage, but you need to be prepared. If you can hire a lawyer to talk about this, I'd recommend doing so.
