Argument: Incest undermines the family and its nurturing relationships
Adam Graham. "Are you ready for incest?" Renew America. October 28, 2004 - In the end, we have laws against incest because society made a moral judgment. We've said that family life is not to be subject to sexual tensions. We believe a daughter should feel comfortable spending time alone talking with her father without having to worry that her father is just another man interested in her body. We've said that the relationship between a brother and sister is to be one of love and trust. We've understood that if we allow incest, families as we know it will cease to exist.
The California Supreme Court concluded in Lockyer v. City and County of San Francisco (2004) 33 Cal.4th 1055 (Lockyer) - We emphasize that our conclusion that the constitutional right to marry properly must be interpreted to apply to gay individuals and gay couples does not mean that this constitutional right similarly must be understood to extend to polygamous or incestuous relationships. Past judicial decisions explain why our nation’s culture has considered the latter types of relationships inimical to the mutually supportive and healthy family relationships promoted by the constitutional right to marry.
Sociologist Vikki Bell was quoted saying in a 2007 BBC article: "Society has long relied on the family unit as its basis. That's why it has been so important to keep family roles clear."
David Archard. Sexual consent. Limits of Consensuality" (Book). 1997 - The damage incest allegedly does to the family can be appreciated by our seeing the family as having both intrinsic and instrumental worth for its members. The family supplies to its members a valued sense of identity, belonging, together with unconditional love and support. Or at least an idealized family, to which many, if not all, actual families approximate, does so. At the same time the family serves as that institution whereby individuals are socialised and educated into the possession of those capacities, aptitudes, and outlooks required of a citizen and functioning member of a society. It is normally thought not only that does the family fill this role well, but also that no other agent of socialisation could do so as effectively and without unacceptable costs.
Given that the family does have this value incest is to be deprecated for subverting it. Incest does so in a number of ways. In induces conflict and instability in family relations; it breaks down the relationship of trust that must exist between adult and child if the former's socialization of the latter is to be effective; it denies the child the opportunity to develop into independent maturity within a context of bounded intimacy and affective identification with significant others; it subverts the terms of the boundary between family and outside world which must exist if the eventual adult is to pass from the family into the larger social world and enter into exogamous relationships. And so on. [author only outlines this argument and does not explicitly agree with the argument, as he outlines refutations and leaves the question open as to the overall conclusion]