I am a man who was coercively assigned female at birth. I also live in California, a state that does not have universal marriage. (I prefer the all-inclusive term "universal marriage" to "gay marriage" or even "same-sex marriage" (though the latter is preferable to the former), because of the lack of a clear legal definition of how people are to be categorized by "sex".) That is to say: I can't get married. I can't marry a woman, because my birth certificate bears the coercively assigned marker 'F' and so the state would probably consider my hypothetical marriage to be a same-sex marriage. I can't marry a man, because my driver's license and passport correctly reflect that I'm male, and so the state would use those documents to tell me that my marriage would be a same-sex marriage. I can't marry someone who is neither or both, because the state would view them as either a man or a woman. The experiences that other trans people have had in the legal system suggest that it's less that there's such a construct as "legal sex", and more that "legal sex" is a fiction that justifies institutionalized bullying. If you're trans, it's not so much that you have a "legal sex" as that a given state agency will designate you as whatever sex causes you the most inconvenience in a given situation.
I can't get married, but even so, I was in a legal same-sex marriage for six years. I got married in 2001 in Massachusetts to a cisgender man with a cissexual body. I was just beginning to come to terms with the reality that I wasn't female, had never been female, that before I was born my brain developed with a set of device drivers that didn't match my hardware. Because all of my legal documentation listed the same sex marker at the time, my legal name was conventionally feminine-sounding, and most (though not all) people who I met perceived me as female, our same-sex marriage was valid.
As far as I know, no state, province or country in the world requires karyotyping before marriage. I also don't know of any locality where you have to drop your pants for the justice of the peace, or show a naked baby picture of yourself, in order to marry. You don't have to get your hormone levels checked -- I had to get tested for syphilis, as per Massachusetts law, but not to make sure that I had two X chromosomes or that I had a certain amount of estrogen floating around in my bloodstream. It's unclear which, if any, of these signifiers government administrators have in mind anyway when they talk about "sex", because the laws that govern us were mostly made at a time when the existence of people like me -- for whom not all of those physiological signifiers align with the same binary sex category -- was an open secret, certainly not one to be taken into account when making law. So what does it mean to outlaw same-sex marriage when there will always be same-sex marriages, at least for as long as it is commonplace for trans people to be repressed and manipulated out of affirming their innate sex until long past the age of consent? To me, it means that same-sex marriage laws -- like laws against abortion -- are not about achieving any concrete social policy goal. After all, all the scientific evidence shows that laws against abortion actually increase the number of abortions performed; anyone who wants to reduce the number of abortions should work to keep or make abortion legal. Likewise, while I don't know that marriage discrimination laws actually increase the number of same-sex marriages, they are intrinsically unenforceable (since the criteria used in practice to issue marriage licenses correlate with but do not tightly conform to individuals' actual sexes), and unenforceable laws usually have a purpose other than their advertised purpose.
This is not a essay about transphobia, or about being trans. I'm offering this backstory to show where I'm coming from when I think about the issue of universal marriage. Restricting marriage according to the sex of one's partners only makes sense if every human can be unambiguously assigned one binary (male or female) sex according to objective criteria. Not every human can be assigned a sex this way; we trans and intersex people are living proof of that. We are not corner cases who the legal system can safely ignore: each of us counts for just as much as any non-intersex person who has a cissexual body. We are just as valid and just as real; laws that make no sense when applied to us are laws that don't make sense. Thus, sex-based restrictions on marriage are little more than institutionalized bullying -- permission for the state to grant and deny privileges to people arbitrarily.
I also object to the exclusive focus on marriage equality that's characterized a lot of the mainstream gay and lesbian movement, at least in the US. It leaves behind all those for whom legal marriage is not the most pressing concern. That doesn't mean that I will co-sign the use of marriage discrimination as a way to devalue or subordinate queer people. And yes, unequal marriage does subordinate us, for reasons explained well by my colleague Christie Koehler, among many other people.