The Malcontent takes Santorum apart in a long, but worthwhile post.
A few graphs:
Santorum has still not coherently articulated how the marriage of another discrete couple affects his own marriage personally. Does it cause him to become divorced? Will it alter the amount of alimony he would pay in the event that he became divorced? Does it alter his joint-filing status for tax purposes? Gay marriage merely opens an institution to couples for whom it was previously denied. Spinning an answer with bogus history is a rather thin veil for bigotry.
Santorum also refuses to contemplate the potential benefits of gay marriage the conservative argument for marriage. Efforts to constantly marginalize and stigmatize will lead to more loveless male-female marriages entered into by gays, and the largely inevitably infidelity, the bringing of STDs into the home, and divorce.
I await Santorum's views on how such a broken home qualifies as an "optimal place" for children simply because it was established as one-man, one-woman in such square-peg, round-hole fashion.