“At this point, liberals tend to throw up their hands. If both parties are consenting adults and the genetic rationale is bogus, why should the law get involved? Incest may seem icky, but that’s what people said about homosexuality, too. It’s all private conduct.”
The conservative view is that all sexual deviance—homosexuality, polyamory, adultery, bestiality, incest—violates the natural order. Families depend on moral structure: Mom, Dad, kids. When you confound that structure—when Dad sleeps with a man, Dad sleeps with another woman, or Mom sleeps with Grandpa—the family falls apart. Kids need clear roles and relationships. Without this, they get disoriented. Mess with the family, and you mess up the kids.
Mohler ends on this note:
"That’s a pretty fair summary. Of course, the Christian argument goes much deeper than the merely conservative argument, affirming the fact that, with exacting precision, God has spoken to the sinfulness of such behaviors — specifically condemning both homosexuality and incest. In other words, Christians move the question from mere wrongfulness to sinfulness and place all issues of sin within the biblical account of sin and redemption.
"It is extremely revealing that, for many of our fellow citizens, incest may merely 'seem icky.' And yet, all around us are folks who, with a straight face, deny the inevitability of this slippery slope."