I confess that as a philosophy undergrad I’ve never read Thomas Aquinas’ treatments of ethics. I hated my intro to ethics class which was designed around the work of Karl Rahner, whom at the time I found not just incomprehensible but also deeply irritating, and I ended up avoiding ethics-related electives.
So I am surprised to see that Aquinas explains in the Summa Theologicae (supplement Q 54 A 3) did not only insist that incest is wrong because of possible birth defects. Instead he gives three arguments why incest is contrary to natural law:
The first is the least convincing, that it messes up the proper family relations like brother / sister / father / spouse. This reason strikes me as being tautological: incest is wrong because a daughter should not be a wife to her father.
The second argument is more interesting: incest is wrong because the family is a place where people are supposed to be learning the virtue of chastity, that is, subjugating lust to charity which values people not for their sexual attractiveness but for themselves. The first time a boy learns to appreciate a girl for who she is and not for her sexual attractiveness is in his relationship with his sister. If not for the taboos against incest, this would not be possible. To use modern language there would be no “safe space” where men and women can learn to subject their sexuality to the deeper bonds of charity. (Later in the section he cites Maimomedes, the a Jewish scholar who was almost a contemporary of Aquinas, as the source of this argument.)
Third: incest is wrong because marriage is meant to expand the circle of friendship between families and therefore promote peace and human solidarity. It is funny to think that rules against miscegenation are modern affair that would not have made sense before the 18th century or so. Of course, marrying outside the faith would have been quite against the rules for Aquinas, unless such a marriage was likely to bring the non-Catholic partner into the church, which also would have been seen as a way to increase the bonds of friendship and solidarity.
I bring this up because apparently Cosmo is now promoting incest. Every few years there are rumblings about making some perversion or another mainstream, to see if it sticks. When I was a teenager you could find flyers from the Man Boy Love Association all over Boston wherever kids would hang out and read sympathetic stories about true love between teenage boys and older men in trendy magazines. Fortunately the Catholic sex abuse scandals reminded everyone just what a bad idea that is, but the idea will come back around. Likewise you find incest stories coming up every few years and reprobates asking “well, with contraceptives, why not?” without regard for what would happen to a young psyche being treated as a sex object in her childhood home.
One of the ironies of the current age: we seek to tear down all sexual taboos and then are horrified by the results.