Sex outside marriage is wrong. Incestuous marriage is wrong. Therefore, incestuous sex must be wrong, whether inside or outside marriage. People should remember that sex and marriage are not about individuals. They are about families. 'The ceremony of marriage was intended to be a bond of love between two (families of different) surnames, with a view, in its retrospective character, to secure the services in the ancestral temple, and in its prospective character, to secure the continuance of the family line.' (Book of Rites 44.1) As marriage is between two families, two individuals from a single family cannot ethically marry, and thus incest is wrong.
Because my morality revolves around success, causing the chance of bad recessive genes to crop up is shooting oneself in the foot.
Also, it's really really REALLY messed up. Boku No Pico, FATAL, and Squad Broken levels of messed up, with some End of Evangelion thrown in for good measure.
*cough* Unless they're identical twin girls and I'm nearby. *cough*
Then it's okay.
Consensual sex is consensual sex. If all parties consent, then what "wrong" could they be doing?
An argument based on on religion is not a logical one by definition. "Someone told me it was wrong, therefore it's wrong", is not a very effective means of debate for everyone who is not part of your religion.
An argument based on the possibility of a retarded child being conceived does not have a basis in morals.
There is no rational reason for keeping laws or taboos against consensual incest that is consistently applied to other relationships. Personal disgust or religion is only a reason why one person would not want to personally engage in what I call consanguinamory, not why someone else shouldn't do it. An adult should be free to share love, sex, residence, and marriage with ANY consenting adults. Youthful experimentation between close relatives close in age is not uncommon, and there are more people than you'd think out there who are in lifelong healthy, happy relationships with a close relative. It isn't for everyone, but we're not all going to want to have each others' love lives, now are we? If someone thinks YOUR love life is disgusting, should you be thrown in prison?
Some people try to justify their prejudice against consanguineous sex and marriage by being part-time eugenicists and saying that such relationships inevitably lead to “mutant” or “deformed” babies. This argument can be refuted on several fronts. 1. Some consanguineous relationships involve only people of the same gender. 2. Not all mixed-gender relationships birth biological children. 3. Most births to consanguineous parents do not produce children with significant birth defects or other genetic problems; while births to other parents do sometimes have birth defects. 4. We don’t prevent other people from marrying or deny them their reproductive rights based on increased odds of passing along a genetic problem or inherited disease. It is true that in general, children born to consanguineous parents have an increased chance of these problems than those born to nonconsanguineous parents, but the odds are still minimal. Unless someone is willing to deny reproductive rights and medical privacy to others and force everyone to take genetic tests and bar carriers and the congenitally disabled and women over 35 from having children, then equal protection principles prevent this from being a justification to bar this freedom of association and freedom to marry.
Some say "Your sibling should not be your lover." That is not a reason. It begs the question. Many people have many relationships that have more than one aspect. Some women say their sister is their best friend. Why can’t their sister be a wife, too?
Some say “There is a power differential.” This applies least of all to siblings or cousins who are close in age, but even where the power differential exists, it is not a justification for denying this freedom to sex or to marry. There is a power differential in just about any relationship, sometimes an enormous power differential. To question if consent is truly possible in these cases is insulting and demeaning.
Some say “There are so many people outside of your family." There are plenty of people within one’s own race, too, but that is no reason to ban interracial marriage.
Two consenting adults who love each other is not my business however strange their relationship may be. But they'd better use birth control of some sort, the results of inbreeding can be quite bad... Although we do it all the time to animals for selective breeding, and the royal families of yesteryears liked it at lot as well.