Because it's just sick and wrong.

Morality is a man made construct. It doesn't exist on its own. Incest is present in animals and I'm guessing in pre-historic societies too. But they also had other bad practices too like rape, pedophilia, etc.
In a just society, where no one is exploited, where there is no age or power imbalance, such an act will not be unethical or immoral like you said.

What is morality? If you commit it and don't know, then most people today would say, ugh, so what. The ancient Greeks, Oedipus, would punish you. It is a man made construct and not good scientifically for reproduction of the species. AND, personally, i would gag to do it. But if it isn't rape in any form, I'll keep my mouth shut.

Brother /sister marriages were common enough in some "royal" cultures, (for ex. ancient Egypt). Apparently their God's didn't have a problem with it. Although recessive genetics can be a b****.

Equine, canine and poultry breeders call it line breeding and it has caused genetic disorders. Ever bragged on your purebred puppy without questioning how it was produced?
First cousins marrying, for example Queen Victoria's progeny promoted hemophilia in her line. I believe whether something is moral or immoral has more to do with what particular society and time you are in.
We invented religion to help enforce rules against conduct that was seen as negative. We invented the gods for our own purposes. Back before we had science it could actually have some positive benefits to society. Or not.

That was the only way isolated tribes multiplied - immorality came later as developed.

We end up with far too many stupid people when they keep screwing their siblings....

Good question! A stellar example of why I love this site! And great responses. Thoughtful, intelligent. I have to throw my vote in with those who state morality is subjective to the time and culture it's being judged by. Incest was completely acceptable in the past for specific reasons. But I think it evolved into an archaic practice, and then demonized, again for specific reasons, such as the high chance of unfortunate offspring. Most of us, including myself think it's pretty sketchy in today's culture. I'd like to blame just religions for the taboo, but there are other, equally important factors.

When it breaks down, morality isn’t that moral unless there’s a fundamental backing in reality. Modern day morality might ask if no one else is effected then it’s neither moral nor immoral. On a purely individual basis, it could depend on the situation. If no harm is done to the individuals involved, how could it be immoral?
But societal morality is at least partially by example. If behavior on a whole would be detrimental, and inbreeding is definitely detrimental to the overall health of a group despite some potential for positive gains, then it could be considered immoral based on established norms or by setting bad example. If everyone or at least a significant percentage of a population indulge in such behavior it would hurt the health of the population as a whole. If a society establishes a rule overall everyone should follow it. Emmanuel Cant made a philosophical argument that morality existed based on a somewhat different argument but I feel like this is where it might have evolved.

1.older members of a family have a position of power over younger members and abuse this power when starting a sexual relationship. This causes psychological harm.
2.It is not prudent to take the of a procreating, that would be at a genetic disadvantage and intensify genetic weakness in their gene pool.
3.Taboos exsist for many good reasons; maybe others can come up with other good reasons.

Yes!

If you made it between consenting adults who weren't going to procreate and possibly create problems with the genetics of their offspring? Possibly okay.

But prior to adulthood? Never - no way no how.

And there lies the problem. If it's ok between adults - kids are going to say "why not us?". And some adults are going to look at kids... and... yup yikes. There you go. Sexual abuse.
(Which already happens a shit ton).

I'm ok with leaving incest taboos just as they are for that reason.

Plus it squicks me hard - and I'm find with that. Whether it's from a Roman Catholic Upbringing or a Genetic Survival Instinct? I don't care.

(Also 2017 - how did this post reactivate? Holy hell... ) lol

You forgot the third option. Amoral.

Incest is generally inadvisable.
The Bible for the benefit of your Christian friend, is peppered with references not judge.

Incest isn't kosher because your kids will come out retarded.

If you question whether or not incest his moral or immoral, imagine having intercourse with your parent and you tell me. Children are born into this world with absolutely zero social networks and rely exclusively on their parents to provide safety. to betray that and use them as breeding stock is simply disgusting.I don't give a s*** what your culture says.