(this is US-centric)
Simply put, different States (and, in States like Texas, even different parts of the State) and the Feds all have different and often contradictory requirements for marriage.
There is no state or federal law that defines male or female. There are administrative rules for the State DMV and Social Security, for example, but those rules are not binding outside of the agency for which they are for - and they can be changed at a whim of the government.
So, whether you are in a legal opposite-sex marriage is entirely up to the courts. Courts have decided that the birth certificates matter, the surgery matters (I do not know of any cases where a post-op male has had a court decision for or against relating to marriage - all the decisions I know about are MTF), the DNA matters, or the original birth certificate (even if an error was corrected) is what matters. Crossing from one jurisdiction to another changes the rules. The Feds could have different rules than the states (I know of no federal case that challenged a marriage yet - just state cases).
Generally, if all legal documents agree and if the person has had surgery, post-op females are generally recognized as being legally able to marry a cis-male, with a handful of states dissenting. But in all these cases, they are one court decision away from losing their married status. Married couples where someone transitioned, sadly, have no security in the US. It's even more confusing for a post-op male, as there just haven't been many cases.
Finally, even if your marriage is legal where you have it, that doesn't mean another state (such as your home state) will recognize it. Each state is different, and, thanks to DOMA, they don't have to respect each other's marriage licenses.
One more thing: That the state accepts a driver's license doesn't mean that they accept the sex on the driver's license for the purpose of marriage. That they issue a marriage license or certificate based on the driver's license (or any other document) also doesn't mean the marriage is legal. It's a sad state.
I agree with others that suggest an attorney. And you should find a way to see his birth certificate, not out of lack of trust, but out of realization that the wrong letter on it will mean you have to take a lot more legal protections in a lot more places.
I know "nobody will likely question it, so it won't matter." But you will find yourself guilty of tax fraud, being unable to be with your spouse if your spouse is dying, and tons of other bad things if someone does decide to challenge it and you lose. So unless you're marrying someone who never pissed *anyone* off, I'd do my best to cover my bases.
That said, I had no problem marrying my wife. I'm willing to go to jail for my love, and accept whatever other consequences, should someone challenge it - but they will get a fight from me, as I won't go quietly. And I'm looking forward to the day when we can get past trying to figure out who is male and who is female so we can make sure the gays don't marry...unfortunately even straight post-op people are caught in that cross-fire. But if you want legal security, a marriage with a transitioned partner, sadly, isn't it right now. I'd listen to your heart instead.