For them, same-sex marriage is about facilitating these joyful moments. Advocates for traditional marriage focus on happily intact nuclear families with a father and a mother.
Their worry is that declaring gender irrelevant to marriage will undermine the web of social expectations that support such families.
This is all a bit odd. In theory these debates are about the law of marriage. That law, however, makes very little difference for happy families. To be sure, this is not entirely true. Tax laws confer a benefit on (some) married couples, and there are a few other legal benefits that the blissfully wedded can claim.