It's ironic, but the best explanation for Santorum, the Catholic bishops, and others of their ilk is actually found in what they all discount and oppose - evolutionary biology. You see, humanity is not innately monogamous. Monogamy is a cultural phenomenon, and even in the Western world, we have officially sanctioned "serial monogamy" (see: New Gingrich.)
Evolutionary pressure is on the males of our species to spread their seed around as much as possible. Evolutionary pressure is on the females of the species to lock down a single good male as the provider, but maybe fool around on the side for some better genes from other males. Evolutionary pressure thus falls upon the males to ensure that while he is allowed to fool around as much as possible, the female for whom he provides is primarily faithful only to him. After all, if he's on the hook for taking care of her children, he'd best get something out of it too.
Enter the dictates of many religions, especially Abrahammic ones.
This isn't to say that the Catholic bishops, or any men of a religion that demands monogamy from women, is something they "can't help." On the contrary, part of what makes us human and distinguishes us from lesser animals is our ability to separate ourselves from our baser instincts. (Usually. Again see: Newt Gingrich.)
The insidiousness is when they are unable to overcome that instinct and instead cloak it in the dictates of culture and religion, relegating women to second class citizenship because it is the only way they feel they can "control" the procreative process.
So this is why we have supposedly celibate religious men so determined to interfere in the rights of women they are themselves not allowed to touch. They have the God-given calling to control the reproduction of women, on behalf of all other men worldwide.
One reason I no longer believe in organized religion is that, in order to overcome our instincts that drive us to make selfish decisions, we have to let go of them. I am monogamous by choice, because I respect my partner and I demand that same respect from him. I also respect the decisions of others to recognize and embrace their instincts, being sexually active with multiple partners (provided they're being safe and consensual about the whole thing.)
But frankly, it's none of my damn business what other human beings do with each other. It does not impact my life or my decisions.
I want to live in a world where every baby that is born is wanted and welcomed, and to me, the best way of achieving that is by preventing as many unwanted pregnancies as possible.
A question I'd love for someone to pose to Santorum: Does he feel that a wife has a right to refuse her husband sex if she is unwilling to birth another child? Since contraception is out of the question, of course.