Those who argue against gay marriage insist that the only marriages between men and women are legitimate, but what about those who are not quite male or female - at least not according to the definitions usually employed? Defining marriage in terms of sex begs the question of how we define the sexes - how we define who is a "man" and who is a "woman."
This is no idle question - there are people who don't match the traditional categories. In the universe of traditional Christianity (and those who can't be bothered to extend their perspective beyond a dictionary), the categories of male and female appear distinct, but in reality there are indeterminate cases which don't fit.
One of many reasons why this argument against same-sex marriage is ineffective is that it assumes that the clarity of the biological concepts translates into corresponding social concepts. Insisting that marriage can only be between a man and a woman causes problems because the nature of "man" and "woman" aren't always as clear and defined as we tend to think that they are.
Those who have undergone sex change operations have the genetic code of one sex, but the physical characteristics of the other. Whom should they be allowed to marry? On December 31, 2002, an Ohio court ruled that a heterosexual couple could not marry if one member is transsexual. Jacob Nash had been born female but changed his sex and obtained a revised birth certificate from his home state of Massachusetts. Despite this, Ohio courts refused to let him marry his partner, Erin Barr.
Judge Diane V. Grendell agreed that Ohio had to accept Nash's revised birth certificate, but that wasn't the same as accepting Nash as a male according to Ohio law. Grendell relied upon Webster's New College Dictionary to define female as "the sex that produces ova or bears young" and male as "the sex that has organs to produce spermatozoa for fertilizing ova." According to this Nash was defined as a female and the marriage was prohibited based upon the state's ban of same-sex marriages.
If we accept Grendell's reasoning, then we must conclude that Nash can only marry a male - even though, by all outward appearances, Nash is also male. How many, especially among religious conservatives, would treat a marriage between Nash and a man as a heterosexual union?
If a woman undergoes a sex change operation and acquires the physical characteristics of a man, would it be legal for this person to marry a man or a woman - and why? If a man undergoes a sex change operation and acquires the physical characteristics of woman, would it be legal for this person to marry a woman or a man - and why?
What about those who have the condition known as "testicular feminization"? They have one X and one Y chromosome, making them genetically male. Because they are resistant to male hormones, though, they have some or all of the physical characteristics of a woman. When "complete," the result is a person who looks completely female; when "incomplete," the degree of sexual ambiguity can vary greatly.
Whom should such people be allowed to marry if marriage is defined as being between a "man and a woman"? Their chromosomes are those of a male, but their physical characteristics will make them look completely or partially female. Would it be a gay marriage for a person suffering from complete testicular feminization to marry a man? If not, when would it start to become a gay marriage for someone with incomplete testicular feminization?
This is a type of sorites paradox. Traditional sorites paradoxes involve asking how many grains of sand you have to remove from a pile before it stops being a "pile," or how many pounds a fat person has to lose before they are no longer "fat." These are paradoxes because they involve characteristics which are vague - it's not clear where a pile or fatness begins and ends.
What is a Man? What is a Woman?
Most will assume that "male" and "female" are non-vague characteristics, and that is true much of the time; yet even biology can become vague and sex isn't just a matter of what chromosomes you have. Socially, we don't distinguish male from female based upon the genetic profile because we don't have access to that. Instead, we take a look at their physical characteristics: do they look stereotypically female or male?
To what degree are we justified in using these markers for distinguishing male from female, though? Are they a reliable basis for social policy and for deciding who can marry whom? This won't be a problem for most couples, but a just social policy must be constructed so that the indeterminate cases are not treated unjustly. Deciding who can marry whom isn't always a simple matter of determining what sorts of internal organs a person has.