What's Love Got To Do With It?

Please explain what letting GLBT couples get married has to do with the well-being of families!

From today's Washington Post:

The president drew applause when he reiterated his long-held stance that marriage is “a union between a man and a woman,” adding, “I believe it’s a sacred institution that is critical to the health of our society and the well-being of families, and it must be defended.”