Americans and their institutions have rapidly transformed their views on gay marriage in recent years. But some 40% of the country remains opposed, and voters in state after state have banned such unions.

It is in this unsettled moment that President Barack Obama weighed in with his personal view that he now supports the rights of gays and lesbians to marry, after earlier having supported only civil unions. How Mr. Obama's changing position is received will go some way toward determining where the country ends up on this contentious issue.