The incest question is interesting. Let's tease apart its elements:
Reasons for preventing (or at least discouraging) incest:
- Is incest a problem because of the risk of birth defects? If so, does that mean genetically unrelated family members are "fair game"?
- Is incest a problem because of the psychological issues that may come about? What about people who did not know they were related?
- Is incest a problem because of the power dynamics that can be involved?
- Is incest a problem for religious reasons?
- Is incest a problem because we find it distasteful, regardless of our source (social, religious, etc.) for this distaste?
- Are there multiple problems with incest?
If we can decide why we don't like incest, we can start to think about what constitutes incest that should be prohibited or discouraged.
- Do we merely want to prevent offspring? If so, sterilized/non-procreative/asexual couples are not a problem
- If there are other problems with incest, what about couples who do not have sex?
Finally, there's the question of the effect of availability of marriage on whether incest will occur.
- Can incest occur if people are not married?
- Can related people be married and not engage in incest? (I know a couple who did consumate their marriage on their honeymoon, and never had sex again. They are still happily married after many decades.)
- How do we know whether a couple is related? If one partner - or his or her ancestors - were adopted, the couple could become married without knowledge of their ancestry. (see http://www.nbcnews.com/technology/gadgetbox/anti-incest-app-built-iceland-college-students-1C9392483 for one solution)
To sum up:
- What's bad about incest?
- What constitutes incest?
- How do we reduce the risk of harm?
With that: my answers:
Closely genetically-related people have higher risks for recessive traits that may be problematic. Genetic counseling and, when warranted, contraception can address this issue for fertile heterosexual couples. Additionally, there can be harmful psychological and power issues in a sexual relationship between closely-related persons. This can be addressed by counseling. Finally, there are religious and social issues around incest, which can be addressed by religious instruction and the imposition of social mores.
For couples who love each other, do not have a history of familial contact between each other, and who are incapable or reproducing, it would seem incest has few genetic and psychological risks.
That said, I am not interested in a sexual or romantic relationship with any of my relatives, and I would think that those who are interested constitute a small fraction of society. It would be more helpful to address the issue by reducing harm than by stigmatizing such couples, as the latter would merely increase the chance that the relationship would be carried on in secret.
edited to correct typographical error