Many people who oppose same-sex marriage base their opposition on their belief that the purpose of marriage is to provide a legal structure in which to raise children, preferably of their own biological origin.
But I believe that issues concerning children and those concerning marriage should be viewed as separately for a couple of reasons.
One is the fact that people marry for a wide variety of reasons these days with almost no one marrying for the single purpose of raising children. People most often marry now for love and because they want to share their lives as a legally and socially recognized unit. And though many people would be happy to just live together to achieve those goals, they enter into legal marriage in order to gain the myriad legal benefits that come with making their union official.
Though most people do have children, it’s almost never their sole reason for getting married in the first place. It’s not as if couples who love one another but who don’t want to or can’t have children decide to simply remain friends because of that fact.
People who have no intention of having children, along with infertile people and people past reproductive age get married all the time, and no one is clamoring to remove their legal rights to do so — as long as they are heterosexual, of course.
Secondly, the law no longer distinguishes between children born to married parents and those born to unmarried parents. “Illegitimacy” has not been a legal status for non-marital children since the late 60s. The law now focuses on how children relate to their parents, rather than so much on how their parents relate to each other.
Hence, this makes opposition to same sex marriage for the “marriage is for having children” reason moot and invalid.