In an interview on CNN, he explained in particular why he does not regard the civil unions as equivalent to the straight marriage. We will not say the opposite, that's exactly why we want the marriage!
On this subject, he thinks that the gay marriage would damage the society.
"I don’t think all domestic relationships are the same as traditional marriage. Marriage between a man and a woman is something that should remain elevated socially, culturally, and practically, legally, morally in our society."
"I think society devalues traditional marriage by saying all other domestic relationships are the same as traditional marriage, you then dilute and devalue traditional marriage."