Why is Rick Santorum's marriage so weak?

Rick Santorum gets the feature in this weekend's New York Times magazine:

The article does not dwell at length on Santorim's controversial views of gay marriage, but when the writer asks him if he feels gay marriage threatens his own marriage, he answers quickly: "Yes, absolutely. It threatens my marriage. It threatens all marriages. It threatens the traditional values of this country."

Is Rick Santorum's marriage really that fragile? Is his wife going to leave him for another woman if it suddenly becomes legal for two people of the same sex to marry? Is his belief in the power or marriage so weak that it will fall apart if even more people can celebrate the bonds of their love? Someone help me understand.
All I know is that my marriage isn't going to change because of what other people do. It makes me wonder if anything is wrong with his.