In this chapter, I argue that we can make sense of moral norms against consensual, adult incest by appealing to the value of familial relationships and the potential for sex to damage them.Viewing sex as unconscionable between family members helps to enable the loving intimacy normally associated with family relationships. Therefore, there is good reason for incest, even when consensual and between adults, to remain taboo. That being said, I argue that there is insufficient legal justification for all consensual, adult incest to be criminalised. I examine four potential arguments in favor of consensual, adult incest being illegal, and conclude that none of them succeeds in justifying a total legal prohibition against it.wrong if it is between consenting adults? From now on in this chapter, the term 'incest' will refer to consensual sex between close adult relatives. I will use 'consent' as Section 74 of the Sexual Offences Act 2003 defines it: "a person consents if he agrees by choice, and has the freedom and capacity to make that choice". In order to focus the discussion, I mainly consider sex between consenting adult siblings and between parents and their adult-children, with the assumption that the sex began in adulthood. Sex between parents and their adult-children or between sibling and sibling is almost universally taboo (Rosman et al. 2009: 101). It is also, in the UK and many other places, illegal.That consensual adult incest is illegal in Western, liberal states makes it unusual, because, as Tatjana Hornle notes, its illegality seems to go against a general legal principle in these states "not to target consensual sexual acts between adults" (Hornle 2014: 78). However, whether or not it ought to be legally prohibited is rarely questioned, including by philosophers, who have paid relatively little attention to the issue.In this chapter I argue that there is insufficient legal justification for all consensual, adult incest to be illegal. This is, at least in part, because incest can take place in quite different contexts, and different cases might raise quite different issues. For example, it is often a product of what's known as "Genetic Sexual Attraction" (GSA): the phenomenon of experiencing sexual feelings for a close relation from whom you were separated at birth but with whom you were reunited as an adult (Colman 2015). However, there is nonetheless good reason to retain a strong social and moral norm against all incest. This is because the perception of sex as unconscionable between close family members helps make the particular kind of loving intimacy normally associated with families possible. First, the norm against incest-if adhered to-removes the possibility of sex damaging the relationship; and, second, it allows family members to engage in emotional and physical intimacy free from suspicion that the intimacy is code for a sexual overture.