After putting forward a thoroughly tongue-in-cheek argument as to why the courts should invalidate a law prohibiting incestuous marriages, Carl Trueman points to the flaws in a system of sexual morality based solely on consent:
[T]he notion of consent is arguably meaningless by itself as the arbiter of legitimate sexual and marital relationships because of the potential for manipulation, coercion, and abuse in a situation where there are deep-rooted and unequal social power relations (e.g., the president of the United States “not” having sexual relations with a besotted young intern or . . . a parent and an adult child contracting a marriage). . . .
Incestuous marriages could well be where the use of consent as virtually the sole basis for sexual morality will founder. These marriages will be coming to the courts over the next few years. They might even make it to the Supreme Court. And they will—or at least should—thereby bring to the fore the philosophical and legal complexities of the issue of consent. As it stands, there is no compelling reason within the philosophical framework of our current sexual morality and marriage laws why . . . incestuous unions should not be contracted. . . .
Do not misunderstand me. I abominate the very idea of incest and contemplate with horror a society that might sanction it by granting such unions the status of marriage. But I did not make our current laws or the logic of their underlying principles. I’m simply thinking them through consistently as new challenges emerge and wanting to see them applied fairly to all.
Read more on First Things: https://www.firstthings.com/blogs/firstthoughts/2016/09/a-family-affair