Argument: Adult incest can occur after family years
David Archard. Sexual consent. Limits of Consensuality" (Book). 1997 - "The problem with regarding such alleged harms as supplying a sufficient condemnation of incest are twofold. [...] Second, we are left with the cases of siblings separated at or shortly after birth who, on being reunited, experience overpowering feelings of mutual attraction which may lead them into sexual intimacy. Such cases (and there are very occasional cases of parent-child incest along the same lines) are rare but well documented. It does not seem that any straightforward appeal to the damage that these incestuous relationships will or might do to the institution of the family is open to those who insist that, nevertheless, they remain deeply immoral."