As anti-miscegenation laws took hold in an effort to stop blacks and whites from marrying, by necessity courts had to start deciding who counted as white or black. The standard that ultimately emerged – namely the “one drop of blood” rule of blackness – dictated that any trace of black heritage, no matter how remote or invisible, made you black.

Although a legal necessity, the rule amounted to a biological and social absurdity, one that thrust upon many people an identity that made no sense in terms of their bodily appearances and their lives. For example, Susie Phipps grew up, lived, and married twice as a white woman, but was informed in 1983 by the Louisiana courts (in response to a passport application dispute) that she was really “colored” because she had had one black ancestor six generations back.

As conservatives continue to push the Federal Marriage Amendment and similar state-level same-sex marriage prohibitions, there are signs that a new one drop of blood rule is about to emerge – but this time it will be about sex instead of race. Soon, the way this sort of legislation is going, against their will some men will legally become women, and vice versa. It’s already happening in Texas.

Let me back up a bit. The Federal Marriage Amendment, like many of the proposed state laws and amendments, says “marriage in the United States shall consist only of the union of a man and a woman.” Simple, right? No. Sex, like race, turns out to be a lot more biologically complicated than it first appears.

Maybe you think that what makes a man or a woman is genitals. But in practice that’s a difficult way to define sex. Male and female genitals arise embryologically from the same parts. The penis and the clitoris are different endpoint versions of the same organ. Same with the labia majora and the scrotum. And so on. That means you can be born with anything in-between genitally – and some people are. About one in two thousand children is born sexually ambiguous enough that a specialist team of doctors is called to decide on the child’s gender assignment.

This happened to Cheryl Chase, the executive director of the Intersex Society of North America (ISNA). She was born in-between, externally and internally (she had both ovarian and testicular tissue). Doctors first recommended that she be raised as a boy, and for 18 months she was. Then a second set of doctors decided she’d be better off as a girl, so they removed her phallus and had her legally changed into a girl. As it turns out, Cheryl fell in love with a bisexual woman and was one of the first hundred people in San Francisco to marry her “same sex” partner.

Because I’ve worked on intersex scholarship and advocacy for ten years, I also know people who were born sexually unambiguous outside but with the parts of the opposite sex inside. Take ISNA’s director of community relations, Jane Goto. Like other women with complete androgen insensitivity syndrome (CAIS), Jane looked like a typical girl at birth. She grew up a girl and had a mostly female puberty – growing breasts, female fat and muscle patterns, etc. – except that she didn’t menstruate. That’s because she was born with XY chromosomes and with testes, not ovaries. CAIS means a girl’s cells don’t respond to androgens, the “masculinizing” hormones, even though her body is making them. So she develops feminine genitals, including a vagina. Like the great majority of people with CAIS, Jane is a straight woman.

Some people try to tell me women like Jane are “really” men trapped in women’s bodies, but that’s just silly. Jane’s brain has had less masculinization from hormones than my female-typical brain. That’s because the cells of typical women (like me) do respond to the androgens our bodies make. (Both women and men make all of the different types of androgens; the difference is just that, on average, women make less of them.)

I also know a man I’ll call Matthew who contacted me because he had just found out, at age 19, that he had ovaries and a uterus inside. He learned he had XX chromosomes and was born with virilizing congenital adrenal hyperplasia (CAH). Because his adrenal glands were in overdrive, Matthew was exposed prenatally to enough androgens that, even though he had female parts inside, his external genitalia developed as male. Matthew wanted my advice on what to tell his girlfriend and parents. (I suggested the truth.) He also wanted to know if he should follow his doctor’s advice and become a woman – essentially get a surgical sex-change. I asked him if he was a woman. No, he said, he was a man. He had always been a boy and a man – he never had had any doubt. “Well,” I replied, “I don’t think you should let your ovaries or your doctor tell you who you are. I don’t let mine.”

If you don’t go with the genitals, then maybe, like the Texas courts, you think that what matters is your sex chromosomes. If you have a Y, you’re a man. If not, you’re a woman. But there are several problems with that: (1) What are you going to do with folks who have Y chromosomes in some of their cells but not others – a phenomenon known as genetic mosaicism? We’re finding more and more of these people. (2) Why would you tell Jane and her husband that their marriage is “same sex” when it looks and feels like my heterosexual marriage to them and to all the world? Jane, like me, has always been a girl and a woman. (3) Would you really want to tell Matthew he’s a woman? A guy who has been a good son to his parents for nineteen years, who has a straight girlfriend who loves him?

Besides, just having a Y doesn’t make you male even if your androgen receptors are working (unlike Jane’s). The Y has to have a functional SRY gene, and there are some people who develop as women because they have Y chromosomes but no functional SRY. Conversely, the SRY can be moved to an X chromosome, so that you can be a man with XX. Some readers of this post will have this situation and never know it – unless maybe the Federal Marriage Amendment runs its illogical course.

Wait! Test for a functional SRY! Nope; again. The medical literature just reported a case of a man with normal male genitals, complete masculinization, and no SRY. He’s infertile, but surely we’re not going to require fertility for marriage?

Bottom line: any “one drop of blood” marriage test of our biology isn’t going to work to match our real sex and gender identities.

Already the Traditional Values Coalition has accused me of being part of the “Gender Confusion Movement.” But I didn’t make this stuff up! And these people aren’t theories. They’re real people with real lives, people who deserve not to get caught in someone else’s naïve ideas about sex. Yet, if we go the route of Texas, we’re talking about letting (even forcing) doctors and scientists to dictate who can marry whom. Why not instead do what makes sense, and let each person tell us who he or she is, and who he or she will marry?