This is an excerpt from my book Jesus Is Not the Answer to Every Sunday School Question: Book 1: Foundations (pp. 67, 68).

For grins and giggles, I threw out one of the skeptic’s favorite Christian-stumper questions: “Where did Cain get his wife?” (See Genesis 4:17.)

“God made one for him!” came one reply.

Most kids said they didn’t know, but one or two said, “From Adam and Eve.”

“Which would have made Cain’s wife his what?”

“His sister?” One student asked incredulously.

You can imagine the cries of “Eeew! Gross!” coupled with the giggles and squirming that ensued as twenty or so sixth-graders began to imagine marrying their respective siblings.

I again challenged their beliefs; I wanted to see if any of them knew why they believed what they believed.

“What’s the big deal? Why is that gross?”

“Yeah,” one boy quipped. “He didn’t have a choice. He had to marry his sister!”

“True enough,” I replied to my bright young interlocutor. “He didn’t have a choice, but does not having a choice make that decision right?” He admitted that it did not. And with that answer eliminated, the class was stumped. I let them stew a bit longer and then bailed them out.

“It wasn’t a problem for Cain not because there were no other options, but rather because God didn’t say that it was wrong to do so! The prohibition against marrying close relatives did not come until the Mosaic law (Leviticus 18:6–18). Prior to the issuance of the law, it was perfectly fine.

But I wanted the kids to think about why God may have put that prohibition in place. And that brought us back full circle to genetic mutations and how relatives are likely to share the same sets of defective genes, and when those latent defects are present in both parents, birth defects are very likely. These genetic defects are a direct result of sin, and after 3,000 years of marriage and “keeping it in the family,” the risk of serious birth defects was increasing, so God said, “No more of that,” and He declared marrying your close relatives to be, as I told the kids, “icky.”