Society has certain rules be you a religious or not. There are many religions, I'm sure that would welcome the practice of incest. I like to consider myself a liberal, live and let live, blah blah, but I also work in family law. The cases I have seen in 35 years with incest somehow end up to be more one-sided. Again, with the male convincing the female the it's ok. In fact look at all these people on this website that agree with me. I'm sorry but I disagree. If the only person you can find to form a loving sexual relationship with is in your immediate family, you might want to seek counseling.

I think that the general aversion to incest goes back well before Christianity, so it's not just based on a religious taboo. It also seems to be very rare among most higher animals, certainly the primates, although there are exceptions, and sexual curiosity (I.e. exploratory, rather than penetrative sex) often occurs in animal family groups.
When a female animal comes into heat, and emits pheromones, it seems that generally other siblings in the group are not attracted by those pheromones, so there's a biological restriction against it.
One of the main human objections to it is that we generally have a different sort of relationship with close relatives to the sort of relationship that leads to a 'normal' sexual attraction.
That's why the cases where siblings have been raised apart and then happen to meet, not knowing that they are related, and fall in love and have sex, are so interesting, and so hard to condemn. It basically goes back to my point that the relationship you have with the person that you know is your brother or sister, and that you were raised with, is not the sort of relationship; that usually leads to sex.
But, with all the caveats and qualifications that have been mentioned by the poster and others (safe, consensual, adult, etc), then there's no a priori reason why it should be wrong.

How did Cain and Abel, the only two offspring of Adam and Eve, produce children? - Sex with their mother? Or if Adam and Eve did have other female children who are not mentioned, they would have been full sisters of Cain and Abel. Enuff said!

I am going to say immoral because there is no such thing as 100% Safe Sex, they are still choosing to take the risk and have that slim and very rare chance of creating a child between them.

There is no such thing as morality. It's only all about suicidal ethics and whatever state society happens to be in. Currently, I think, anything beyond a first cousin is allowed to be married in the US, so technically incest is legal between 2nd cousins etc. Back in the heyday of middle ages, you can only imagine what interesting relationship dynamics existed. Morality and immorality suppose the existence of a set of universal rules regarding behavior, and I think we all agree that's basically bullshit.

Wealthy and powerful families used to intermarry with their own family members . This often led to health problems for the children they produced , Is it moral to intentionally and with forethought create geniticly sick children ?

Morality is mobile. What may be immoral for us today, may not have been in the past or in the future.
For now, for me it's immoral.

Ewwe wwww

Why would you want to traumatize and ruin a persons life.

There is an inhibition in humans and other mammals to have sex with one whom you have known since infancy. Doesn't have to be related.

Why is it immoral to a theist is the better question. If god only created one couple then he mandated incest for a time. Even without the creation myth science tells us we’re all related. Everyone in the world is no more than 15th cousins to you. It’s all relative, pun intended but sex with close relatives is discouraged by evolution more so than it is by god. I’m not sure any sex act other than Rape can be seen as universally immoral, but it’s certainly unethical and not recommended for all sorts of reasons that one would hope should be obvious with or without a god. When theists ask stupid questions like this or what prevents us from killing/raping make sure to ask them “oh so gods law is the only reason you haven’t fucked your sister/raped/killed yet? Interesting. I’ve raped and killed and fucked as many sisters as I want. And that number is 0”

Moral noun "a person's standards of behavior or beliefs concerning what is and is not acceptable for them to do."
So where does this standard come from?
Our society, for lots of reasons, family structure, inbreeding, and so forth.

Why do so many Atheists FAIL to see these are ALL strawmen arguments from theists?
They believe in a God and a religion with its laws, Morals and precepts.
We do not.
Since they cannot PROVE the God and hence the validity of the Laws and precepts, they instead construct endless strawmen. Evolution, Cosmology and all sorts of moral arguments.

"If there is no God why is safe sex between brother and sister immoral to an atheist?"
No answer to this Proves God, it is a strawman.

I can't vote because the question isn't based in moraity or lack of it. The answer lies in the history of man as we know it.

It goes against our evolution and instincts. Many animals avoid incest in healthy populations. That is not because of god. Humans just added social taboo.

Please tell me, where in the Bible it says it's a sin? I've never heard of such a Bible verse. People are so stupid when it comes to religion. Most practices of the Christian religion, is not even in the Bible. Most of it rationalizations of someone's "interpretation". The fact is, in the Old Testament there are a hand full of direct commandments, and most of them are ignored today.

If children are produced it is bad for the genetic diversity of the offspring. Look at the Amish. They are descended from a very small gene pool and for the most part only marry other Amish, so they have a very high number of genetic diseases.

I define good as that which advances life and evil as that which impedes life. Incest impedes life and is therefore bad.

If it is safe sex, you're skirting the edge of morality and immorality, but on the right side. Barely.

Now, thinking that you have a right to decide that for others is highly immoral and should not be tolerated.

To me it's immoral, because all sex must include complete consent, and I don't think people who are related can give true consent, because of power dynamics, etc.

Inbreeding magnifies genetic defects.

Here's a better question; why do Xians have a problem with incest when the bible advocates it prominently? If you believe the bible is the literal word of god then incest is the ONLY explanation for the continued presence of Man after the flood. Since nothing in the bible claims that god repopulated the world with, y'know, magic, then the only other means available would've been an ongoing exchange of genetic material within Noah's family.

It's Nature's Way of keeping us from having two headed 12 toed babies

Any time close relatives have sex and a pregnancy results, the couple is playing sexual Russian roulette. Recessive genes that can produce disorders exist in almost every person, having accumulated over thousands of years. When close relatives interbreed, the risk of those recessive genes being in both parents and making a disordered child skyrockets. A disordered child is wasted reproductive potential. It is always better for people to find mates outside the family.

As long as they're two consenting adults it shouldn't be a problem.

When playing around with sex, one may become pair bonded. As a teenager, I did with my steady girlfriend. She moved far away with her family. It was devastating for us.

Now think about the issues when one sibling pair bonds and the other does not. You are living with the tension of unrequited romantic love, with someone you will have a familial relationship with for life. This kind of mismatch will really test your patience or your relationship. So sex play with sibling without having a solid bond is hazardous, if not immoral.

On the other hand, consider a couple that falls in love and plans to delay pregnancy. Or siblings / cousins that fall in love and will have (frequent) "safe" sex. Of course, a single contraceptive is around 80% effective annually. (Always using two might be 96%. That means your odds of pregnancy are about 50% in nine years.)

If the children have 25% odds of having one (or more) active recessive diseases, the relationship is far less important than the suffering non-health of their progeny. So I conditionally call this reproduction immoral.

You don't want the tragedy of a cemented pair bond with someone who is likely to produce a diseased child.

So my advice to young people is to get your genetic testing as soon as you can afford to. Compare your test results before getting all hot and heavy. This is essential for siblings and cousins.

Ruins the sister-brother relationship, one that is based on traditional values excluding religious beliefs.

I know in many old cultures it was actually quite common, but due to the health issues it usually lead to collapsing dynasties.