Christianists and social conservatives have argued against same-sex marriage, because children are the central point of the insitution. One reason they have largely lost the argument is that heterosexuals no longer think of the institution that way:
"When I think of marriage, I don't think of children at all. I have them. But with marriage, I think of a husband and a wife, and I don't think it's the children that make it work."
That's a 42-year-old DC resident. There's an argument to be made that marriage should be re-focused on children, but as a purely civil matter, it isn't any more. That's why gay marriage is here to stay. Because, under the current cultural meaning of civil marriage, the discriminatory exclusion of some childless loving couples and inclusion of others makes no sense at all.