Dear Catholic Right:
You know how every time you get into an argument about gay marriage, you ask, “Well, if we allow gay marriage, what logical reason do we have to disallow brother-sister marriages?”
You should really stop that. Here’s why:
If you are an orthodox Catholic, you believe in monogenesis—that is, you believe that all of humanity descended from two original humans, Adam and Eve. You also believe that God commanded Adam and Eve (and by extension all of their children) to be fruitful and multiply. Period. These are non-negotiables according to
infallible decrees by your infallible popes. the infallible teachings of your Church, as Pope Pius XII pointed out in his 1950 encyclical Humani Generis.
In order to be fruitful and multiply, Adam and Eve’s kids had to have sex with each other. Ergo, you believe God commanded incest. God can’t command something that’s contrary to God’s will, and no commandment from God can ever be unnatural.
So there’s no getting around it: by the logic of Catholic theology, brother-sister incest is natural.
Now, I’ve seen some attempts to explain this away. One suggestion is that Adam and Eve’s kids got a special dispensation to have incest, but as soon as there were enough people to make incest unnecessary, it became unnatural. That doesn’t fly for two reasons. First, natural law doesn’t work that way—the whole point is to figure out what’s timeless and unchanging. Nothing that was natural for Adam (or his kids) can possibly be unnatural for us.
The second is that it doesn’t square with Old Testament history, which is full of incestuous unions, even hundreds of years after they would have been necessary. Abraham’s marriage to Sarah, for example, came ten generations after Noah. By that time, Abraham could have married plenty of unrelated women—we know Hagar was willing to make him a baby—but he chose Sarah, his father’s daughter. And that union was blessed in the sight of the Lord.
Catholics can rightly point out that Church law has since proscribed brother-sister unions, and that such unions are therefore, for Catholics, now a sin. But Church law holds no weight in our legal system. This, again, is why you use natural law in your arguments against gay marriage.
So why do we ban brother-sister marriages? Most people will tell you it’s because we don’t want brothers and sisters reproducing, since pregnancies that result from those unions have an unusually high incidence of problems*.
*This wouldn’t have been a problem before the Fall, obviously, because that’s when disease and death entered the world.
But as she ages, every woman is at a greater risk for producing a child with birth defects and genetic disorders. And this risk increases every year of her reproductive life. If a woman gets pregnant at 45, the risk for down syndrome is about 40X higher than it was when she was 25. And I don’t see the Catholic Church arguing against marriage for women over 45, or asking those already married to practice celibacy**.
**Nor should they—I’m just pointing out an inconsistency here.
Besides, we have models for allowing marriage among partners we hope won’t reproduce—several states allow first cousins to marry, but only after they’re too old to produce children. Arizona, for example, allows first cousins to marry if a) they’re both over 65, or b) they get a doctor’s note proving that one partner is infertile***.
***In the twisted logic of “natural law”, these marriages are okay because they’re “ordered” towards procreation, even though to enter one, one partner has to prove incapable of reproducing.
So let’s say you live in Arizona. A brother and sister (let’s say the sister is infertile) come up to you and ask you why, legally, they shouldn’t be allowed to marry. What are you going to say?
You can’t argue that it’s unnatural.
You can’t argue that it’s to prevent a risk to potential children.
You can’t argue, in our constitutional system, that it goes against Church law.