GALLAGHER: Marriage as an important cultural status is rooted in a shared belief that we need to bring together male and female to make and raise the next generation together, and that adults have a serious obligation to make serious sacrifices, including of their sexual life, to make that happen.

Marriage, after gay marriage, is an under-defined commitment to love and caretaking, whose public character and status is newly uncertain. Why love? Why sex? Why just two? What does this have to do with parenting? What other relationships have an equal right to be counted as marriage?

Gay-marriage advocates will work this out, or more likely ignore — with the exception of a few like David Blankenhorn and Jonathan Rauch — these questions in favor of pursuing an ever more energetic strategy of using the power of law and culture to push new expanded equality norms around gay and transgender relationships.

The challenge is that marriage is both a social convention and a legal one.

Current battles are not just over social acceptance but over legal rights.

I guess she's referring to this sort of op-ed by Rauch:http://www.nytimes.com/2009/02/22/opinion/22rauch.htmlAnd while most Americans who favor keeping marriage as it has customarily been would prefer no legal recognition of same-sex unions at either the federal or the state level, we believe that they can live with federal civil unions — provided that no religious groups are forced to accept them as marriages. Many of these people may come to see civil unions as a compassionate compromise. For example, a PBS poll last fall found that 58 percent of white evangelicals under age 30 favor some form of legal same-sex union.

And this from http://en.wikipedia.org/wiki/David_Blankenhorn : “A noted figure in the campaign against same-sex marriage in the United States, his position changed and he voiced support of legalizing same-sex marriage in June 2012.” 

http://www.nytimes.com/2012/06/23/opinion/how-my-view-on-gay-marriage-changed.html

Marriage is how society recognizes and protects this right. Marriage is the planet’s only institution whose core purpose is to unite the biological, social and legal components of parenthood into one lasting bond. Marriage says to a child: The man and the woman whose sexual union made you will also be there to love and raise you. In this sense, marriage is a gift that society bestows on its children.

At the level of first principles, gay marriage effaces that gift. No same-sex couple, married or not, can ever under any circumstances combine biological, social and legal parenthood into one bond. For this and other reasons, gay marriage has become a significant contributor to marriage’s continuing deinstitutionalization, by which I mean marriage’s steady transformation in both law and custom from a structured institution with clear public purposes to the state’s licensing of private relationships that are privately defined.

…

Instead of fighting gay marriage, I’d like to help build new coalitions bringing together gays who want to strengthen marriage with straight people who want to do the same. For example, once we accept gay marriage, might we also agree that marrying before having children is a vital cultural value that all of us should do more to embrace? Can we agree that, for all lovers who want their love to last, marriage is preferable to cohabitation? Can we discuss whether both gays and straight people should think twice before denying children born through artificial reproductive technology the right to know and be known by their biological parents?

So the argument is that "marriage" inherently has a biological component?

There are plenty of people who are married but never have children.