Gay Marriages and the Defense of Marriage Act

Something's been troubling me about this hoo haw over gay marriage and the Federal law, The Defense of Marriage Act.
At heart I'm a libertarian, although with a wider view of what is permissible in the name of the public good than many US such, so my view on the basic idea of gay marriage is as it is with my view of any marriage, any version of sexuality or lifestyle choice. I'm with Mrs Patrick Campbell :
"Does it really matter what these affectionate people do...so long as they don't do it in the streets and frighten the horses."
and that seems to be just a wittier version of earlier views on where my liberty ends and your's begins.
The thing that puzzles me is the detail of the 1996 law :
Marriage is "a legal union of one man and one woman as husband and wife" and spouse "refers only to a person of the opposite sex who is husband or wife."
Well, yes, I suppose so if you want, it is your country and your law, but have you ever actually thought of what is the definition of a man? Of a woman? Ever heard of intersexuals?
Contrary to what many believe, XX and XY (traditionally thought of as male and female chromosome arrangements) are not the only possibilities in humans. There are some 12 possible variations, XXY and XXXY which leads to Klinefelter's Syndrome, something seen in perhaps 1 in 500 or 1 in 1,000 "male" births, depending on who you believe. XO, Turner's Syndrome in 1 in 2,500 "female" births. Not XX and not XY is some 1 in 1,500 of all live births. Further, genes are not the only determinants of sex, hormonal influences during pregnancy have a huge effect on the development of genitalia for example, and some posit on brains as well. 1 in 100 live births is the number of "not standard male/female".
Now I don't know what the US legal definition of male or female is, presumably the blogosphere can help me out as there seem to be more Law Professors out there than any other mode of financing a lifestyle. Somthing to do with the crushing time pressures of academe I guess. It would at least seem possible that there is some ambiguity about how to define it.
So how exactly is this Defense of Marriage Act supposed to work then? Who gets to state who is male and who is female? Will it require DNA tests before the licence is issued?
I have a feeling that the law should not be involved in such detail. There was a case in the UK of a baby born with odd genitalia leading to inconclusive assignation of sex. Raised as a woman, indeed to the point of being a Debutante, later in life, and after inheriting a Scottish Lairdship, she reassigned herself, became a man and married the housekeeper. Photos in later life showed a fine moustache and male pattern baldness, two things usually thought of as unattractive in women, and while there were no children from the marriage there seemed to be no other problems. His prediliction for kilts should be seen as more an abberation of his nationality than his sex.
All of this happened back in the 1950's and the major concern, the major outpouring of disgust, was that it was so dreadfully common to marry one's housekeeper.
Which is, I think, just about the right attitude. What one does or does not have in one's knickers has little import in marriage. Or perhaps who one marries is more important than what one marries.