In an article on Public Discourse, Christopher Tollefsen writes of Aquinas’s three natural-law-based reasons why incestuous unions ought to be forbidden, and one of those reasons touches on an aspect of incest I hadn’t thought of before:
Aquinas’s third reason is that incest is contrary to an “accidental” end of marriage: the binding together of humankind and the extending of friendship. We see this at virtually any wedding when two unrelated families gather to celebrate their new connection. Incestuous unions would have nothing of this sort to celebrate.
This is interesting because it seems to be related to one of the problems with homosexuality. See this quote from John Piper on Romans 1, which I previously posted (with more context) in “Worshipping Images of Ourselves”:
We learn from Paul in Ephesians 5:31-32 that, from the beginning, manhood and womanhood existed to represent or dramatize God's relation to his people and then Christ's relation to his bride, the church. In this drama, the man represents God or Christ and is to love his wife as Christ loved the church. The woman represents God's people or the church. And sexual union in the covenant of marriage represents pure, undefiled, intense heart-worship. That is, God means for the beauty of worship to be dramatized in the right ordering of our sexual lives….
[T]he disordering of our relationship to God is dramatized by the breakdown of that heterosexual union.
Homosexuality is the most vivid form of that breakdown. God and man in covenant worship are represented by male and female in covenant sexual union. Therefore, when man turns from God to images of himself, God hands us over to what we have chosen and dramatizes it by male and female turning to images of themselves for sexual union, namely their own sex.
If homosexuality, contrary to the purpose of sexual union in marriage, is an image of our turning away from God and worshipping ourselves, then incest, as a similar turning inward towards those in our family who are like us rather than an outward turning towards a different family, is likewise contrary to the purpose of our sexuality. We’re to worship outward, not inward, and our sexuality is meant to reflect this.
It has taken me a while to realize how artistic God actually is—that many (surely, all!) aspects of this world, not just sexuality and marriage, exist to ultimately point us to Jesus. Like the beauty of a painter who speaks to us through images in his artwork, God has created and arranged the things of this world to paint for us Christ and His work on the cross. Here’s another example of this from John Piper:
One of the reasons God created bread—or created the grain and the water and yeast and fire and human intelligence to make it, and I mean the really good kind, that’s not mainly air—is so that when Jesus Christ came into the world, he would be able to use the enjoyment of bread and the nourishment of bread as an illustration of what it means to believe on him and be satisfied with him. I believe that with all my heart. Bread exists to help us know what it is like to be satisfied in Jesus.
The temple, the Lord's Supper, the holiness code, marriage, bread, even slavery—look at the painting God has created for us, the images in the story He is telling us, and see Christ, our needed redeemer. As James Sire says in Apologetics Beyond Reason, “As it turns out, our world is so fashioned and has such a history that it is no exaggeration to say that every person is created for Christ and everything created makes a case for Christ.”