Homosexuality, Incest and Morality
There are two aspects of this question that we must distinguish here. One is the question of whether two consenting adults have the right to do as they please. This is an issue of individual liberty, regarding the morality of limits of social interference, which we have established using the criteria of ‘adulthood’ and ‘consent’. This in itself says nothing about the positive or negative moral value of what those two consenting individuals choose to do.
The sexual relationship in which a couple chooses to indulge in dehumanizing and degrading forms of sex, but within privacy and with mutual consent, are well within their ‘rights’ to do so (i.e. society is not justified in interfering with what goes on between them) but that relationship cannot be ascribed the quality of being ‘moral’, because it possesses no positive moral value as such. Sadism and Masochism as character traits only have negative moral connotations. (I'm speaking from a broadly virtue ethical perspective. Utilitarian and Deontological notions of morality are too narrow to distinguish the moral richness of such scenarios.)
Most people while defending homosexuality rely exclusively on the ‘rights’ principle so much so that they totally ignore the fact that a positive moral case can be built for homosexuality as well. American Psychological Association, for instance, states:
“Same-sex sexual attractions, behavior, and orientations per se are normal and positive variants of human sexuality—in other words, they do not indicate either mental or developmental disorders.”
“Gay men, lesbians, and bisexual individuals form stable, committed relationships and families that are equivalent to heterosexual relationships and families in essential respects.”
This can be used to argue effectively that homosexual relationships can have the same positive moral attributes that any heterosexual relationship can have.
When it comes to incest, definitely it comes under the protection of rights principle, but its moral value per se is difficult to judge. As you have identified yourself, sanctioning incest robs the home and family of its safety. For a society which morally values the institution of family, this is one very pertinent reason to disapprove of incest. Secondly, even if safe rearing of children is somehow guaranteed, the possibilities of emotional exploitation in a romantic relationship with family members are far greater than in a relationship with non-family. Responsible societies disapprove of relationships between psychotherapists & patients, and teachers & students, because they can recognize how the role asymmetry can lead one to exploit the other, even in a sincere, consensual relationship. If we are justified in disapproving of a relationship between psychotherapist & a patient for this reason, we have far greater justification for disapproving of a relationship between a parent and a child, even with consent. In specific situations these factors may or may not come to play (siblings who grew up apart, etc) and hence some specific cases may not be subject to disapproval, but generally I believe we justified in disapproving of incest, yet I do not think it is reason enough to criminalize consensual, adult relationships.