Simply put, mutual responsibility towards offspring naturally demands a long-term commitment (at least 18 years) while mutual attraction and erotic desire does not. What we see in the modern world is the fracturing of a very lofty ideal of marriage back into two different kinds of relationships: those which are primarily focused on children, and those which are primarily focused on erotic love. The battle over the institution of marriage is basically a battle over whether which of these two purposes of marriage ought to have primacy.
The answer that the Supreme Court has given by ruling in favour of same-sex marriage is basically a ruling in favour of erotic love. This should surprise no one. It’s the more culturally popular option, and it’s the view of marriage that the vast majority of heterosexuals already subscribe to. It’s also, in practice, the definition that we’ve been using for a long time. The truth is that most of the material and social supports that exist to help parents with the task of raising children are no longer associated with the institution of marriage in any way – and unfortunately, the pro-family groups that could be providing financial, emotional and practical support to people who are choosing traditional marriage tend to waste their resources fighting fruitless political battles instead.