One of my handicaps in life is that things that sound absurd to other people often make sense to me.
For example, Charles Cooper's argument that older heterosexual couples need to be allowed to get married to restrain the fearsome procreative power of men over the age of 55. I mean, I'm not denying that the image is funny, especially since I feel safe in assuming that Cooper is himself over the age of 55 and the justices he was making the argument to most definitely were. But, if you're ok with abstract philosophical arguments about human behavior, it's not unreasonable. Adult human beings have a strong tendency to have sex. An imperfect but widely attempted strategy for controlling that tendency is to let everyone have one "right" person to have sex with while making it "wrong" to do with anyone else. Paul said in Corinthians that it was better to marry than to burn, in the Good Earth Wang Lung gets his sons wives to keep them from going to prostitutes. (Although it occurs to me that Wang Lung was not a real person and his story was created by Pearl S. Buck, so I'm not actually branching out of western culture here). So sure, I guess if marriage exists for the sole purpose of preventing illegitimate children, the marriage of a 55 year old, presumably fertile, man to a 55 year old, presumably infertile, woman has a legitimate role in that.
But the problem is that the exact same logic applies to gay couples. Gay people are capable of procreative sex, after all. Assuming that no one ever has sex that contradicts their orientation is only slightly less naive than assuming people don't have sex unless they love each other and want to have babies together. Of course, this is the modern age and people have expectations about love and compatibility beyond gender that prevents making people get married from being a simple solution to any problem...but if conservatives are going to be going around claiming that making poor people get married is the solution to poverty, and making unwed mothers get married would eliminate gun crime, I don't see how they can deny that making gay people get married would prevent gay unplanned pregnancies.