Gay marriage has been successful in the courts precisely because, given both the actual behavior of married Americans and the prevailing legal logic of many decades, it was impossible to maintain the fiction that civil marriage remained an inherently procreative institution, even if sacramental marriage in some faiths remains so. It took the arrival of gay marriage to wake some traditionalists up to the fact that that religious and secular notions of what marriage is had long since parted ways. But the logic of same-sex marriage requires no leaps beyond what heterosexuals had already made. It fits within the Enlightenment model of civil marriage as a contract. For traditionalists, its logic is such that nothing new is lost....Why? Because if civil unions/domestic partnerships had been the norm, straights would have wanted access to them too, precisely because they weren't marriage. That's what happened in France, for example.
Instead, a culturally influential minority is now included in marriage, so it remains the default way that couples join; religious people are as free as ever to marry in a fully traditional sense; and secular straights retain more traditional aspects and attitudes than they would if they'd switched over to a new paradigm of coupledom.