Marriage and Biology 

Defining man and woman is harder than you might think.

As a biologist I have found the arguments against same-sex marriage misguided. Not because the evidence hints at homosexuality being based, at least in part, on biological roots, but because the same reasons used to keep same-sex (homosexual) marriage illegal could also be applied to what appear to be opposite-sex (heterosexual) marriages. It may be shocking to hear that the sex and gender of every individual in our population does not fit into a nicely defined box that can be labeled male or female, man or woman. Observations that substantiate tremendous diversity in sex and gender within our population pose a biological conundrum: How should we define the terms man and woman, and are they the same as male and female?

Male and female are biological designations used to define one's sex. Generally the two sexes are distinguished by whether they produce sperm or eggs. In order to do so, each sex needs to possess functioning internal and external reproductive tissues and organs.

Yet many of us would have a hard time stripping an individual of their sexual distinction if they were for some reason unable to produce viable and/or functioning sperm or eggs. Therefore, we have accepted individuals as being male or female based in large part on how they look physically. For example, a female would possess a vagina in addition to other female-associated parts, and a male would minimally possess a penis. The development of male and female parts is determined by the genetic information contained within our sex chromosomes. The sex chromosomes possessed by a typical male are designated as X and Y, and for a typical female XX. Although other, non-sex chromosomes play a role in male and female development, the presence or absence of the Y chromosome is critical in cueing the early embryo to develop male or female parts.

It might seem reasonable to assume that all males would possess XY chromosomes and associated male physical parts such as a penis, and females would possess XX chromosomes and associated female physical parts such as a vagina, but this would be an incorrect assumption. It is estimated that 0.05 percent or 1 in every 2,000 individuals in the population do not fit this pattern, and for the United States this translates into approximately 150,000 people or the population of Dayton, Ohio.

Although there are many examples of how a variation in chromosome composition and concordant anatomy can occur, one example is called androgen insensitivity syndrome or AIS. Individuals with AIS are represented in the population at a frequency of 1/12,000. In one form of AIS, individuals can look female externally, lacking a penis and possessing breasts. However, they minimally possess the male XY chromosomes and are born with testicles that can remain in the abdominal cavity. This condition occurs because the developing embryo fails to make functioning receptors that are capable of binding to testosterone, which is needed to complete the development of male-specific reproductive organs and tissues. Without receptors the hormone has no effect.

Thus, the existence of AIS in the population begets an interesting question — are individuals with AIS male or female? If we base the determination on chromosomes alone, then the answer would be male. And if these individuals are male, and married to XY, penis-bearing males, can their marriages truly be qualified as heterosexual? Some would argue that other factors must be taken into account in determining one's sex. After all, individuals with AIS may never know they are chromosomally male unless they seek medical attention regarding painful intercourse that can occur due to a shallow vagina, or their inability to menstruate or get pregnant due to the absence of a uterus, fallopian tubes, and ovaries.

Should one's sex then be based on how they look externally? There are cases of AIS where the individual's testosterone receptors slightly work, allowing for the development of a penis. In this situation does the answer change? Does the individual's possession of a penis and XY chromosomes now make them a male, even though in every other physical regard they could pass in our society as female? And if so, would their marriage to a typical, non-AIS penis-bearing male be valid? Some might argue that depends on the sex the person was assigned at birth. However, what if a sex assignment turns out to not match the person's gender?

Although gender is commonly used interchangeably with sex, gender is determined based on cultural and social mores and refers to definitions associated with man and woman or masculine and feminine. For most individuals, one's sex and gender match. Males would self-identify as men, and females would self-identify as women. However, in the case of AIS, sexual assignment cannot be strictly defined, or might be inaccurately assigned so as to not match gender. What is the marital fate of an AIS individual who has been assigned the sex of female, yet self-identifies as a man, and is sexually attracted to females? Whom will this person be allowed to legally marry?

In 48 of the 50 states that make up our country's union, the identification cards presented to the clerk by the two persons desiring a nuptial must state male on one and female on the other. In other words, the sex of the two "pre-weds" must be opposite. Matthew Staver, founder of the Liberty Counsel, which opposes same-sex marriage, says, "What you're born with is what you are." In essence, marriage legitimacy should be based on birth sex. In 2004 he argued and won a case that nullified a marriage between a female-to-male transsexual and a female. The court ruled that both individuals shared the same sex, each born female and in possession of XX chromosomes. To be consistent with his own tenet, Staver should be arguing against the legitimacy of thousands of "heterosexual" marriages that are in fact unions between individuals possessing the same chromosome composition.

Biological variation is common among all species and exists naturally. I do not believe we will want to establish a list of criteria to determine what defines a man and a woman, and as a consequence control who is fit to marry whom. There is no need to call into question the legitimacy of thousands of "heterosexual" marriages that are really homosexual unions. Instead, we can decide to embrace biological diversity.

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