By Zack Shaeffer
As a recovering Evangelical, with family and friends who are current Evangelicals, I have definitely heard (and perhaps made) the definitional argument before. "Allowing gay men and women to marry would fundamentally change the definition marriage has had for all of human history!"
The rejoinder that I have recently made to this challenge is that while same-sex marriage would have once been a non sequitur, the definition of marriage has already changed in key ways that make it illogical to exclude same-sex couples from the praxis of marriage in Western culture. For instance, marriage has largely ceased to be regarded primarily as a vehicle for property inheritance, a means of producing heirs, or a way to cement economic partnerships between families, businesses, or nations. Women's entry into the workforce, and increasingly competitive earning power, has also made women less economically dependent on men (whether their fathers or their husbands), opened up the patriarchal social structure based on male-headed family units, and provided economic choices and independence that were once available to hardly any women at all. Widely available birth control has also given women and couples the power to choose when they will reproduce, and increased social tolerance of out-of-wedlock births has made terms like "illegitimacy" almost quaint. The waning of illegitimacy as a social and legal concept reveals once again the importance inheritance by legitimate heirs had in the past social structure of marriage. The ability to adopt or to utilize artificial insemination makes it easy for gay couples to have kids and start a family. I am sure more examples could be cited, but the point is that the essential elements cited by the traditional marriage crowd are already open to unmarried people and same-sex couples, whether they are married under the law or not.
In short, the definition of marriage in Western society has already been steadily changing for the past hundred years or more, to the point that today marriage is primarily defined as an elective union, based on mutual affection, between two people who wish to commit their lives to one another. Inheritance, dependence on a single breadwinner, and even reproduction are no longer regarded as essential aspects of the marriage relationship (although they certainly remain important on a individual preferential basis). Therefore, marriage has become a social reality that does not exclude same-sex couples by definition.
This, I think, is the essential reason why Focus on the Family and similar groups fit opposition to gay rights within the complex of resistance to social changes in gender roles and relationships. For them, the family is essentially a male-headed and male-dominated enterprise, which is profoundly threatened by women's liberation and gay rights. If you want to get all Freudian, this all boils down to castration anxiety. For those in society who don't share these commitments to a 1950s, or even medieval, patriarchal social structure, it is difficult to come up with a good logical reason why the definition of marriage should exclude loving, committed, same-sex partners.