A Blast from the Past

From Atrios: Senator Rick Santorum (R-Pa) discusses gay marriage on The 700 Club.
"[T]he consequence is very clear. Marriage loses its significance. People will stop getting married. Homosexuals will not get married; heterosexuals will stop getting married. And that to me is the real threat to the American family and to the culture generally."

Atrios notes that in 1911, Rep. Seaborn Roddenberry of Georgia introduced an anti-miscegenation amendment, saying:
"Intermarriage between whites and blacks is repulsive and averse to every sentiment of pure American spirit. It is abhorrent and repugnant. It is subversive to social peace. It is destructive of moral supremacy, and ultimately this slavery to black beasts will bring this nation to a fatal conflict."

I wonder how those who agreed with Rep. Roddenberry in 1911 explain the failure of his prediction. I'm curious to hear from Sen. Santorum specifically how his predictions will come to fruition. What is the mechanism that will cause this change in our society from one where marriage is a common occurence to one where people will stop getting married?