This question brings up an interesting realization that I had recently about theists. They determine their morality and actions based on a bunch of rules loosely (very loosely) derived from a bronze age storybook, not from an understanding of right and wrong. If Atheists don't use their rules, they don't think we can have morals, which is why they keep asking us for our set of rules. They just don't understand that morality relates to right and wrong, not to a set of rules, and they don't really have a definition of right and wrong beyond their rules.

I had fun in a xtian forum by asking theists for their definition of good and evil, and NOBODY could come up with one. Some people would say "god is good", to which I responded "and god nearly exterminated humanity, so should we measure our goodness by how many people we have killed?". They just can't conceive of actual morality.

Don't. Fricken Hanoverians should be enough of an example. Safe sex? No, you mean safer sex. It's similar to the term "less than lethal" in law enforcement.

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.

I voted “immoral.” In the rare instance of adult, consenting adult siblings engaging in sex, I don’t have an issue and it is not necessarily immoral. This scenario is more likely and why I voted “immoral.” If either or both cannot consent (age, capacity) or an unbalanced power dynamic, then it is immoral. But I’m left with why in the heck would siblings want to engage in sex?

I think it is immoral if it is parents and children, but not between two siblings if they are adult and consenting. I wouldn't do it, but I think it is okay between brother and sister and it started in adulthood and both are of similar age. When power dynamics are involved, it gets weird and wrong. So there have to be caveats.

Cain and Abel can best answer the moral aspect, other than that, to my way of thinking, adult, consenting sex should be shared freely among people...if you are going to believe in a god, then accept it as a gift...

Sex within a family system between siblings is neither moral nor immoral. It's damaging and psychologically harmful.

No vote from me, because "it depends". "Is incest [im]moral" is really the wrong question. The real question is, are the parties involved able to give informed consent and is there a reasonably balanced power dynamic?

In the case of two consenting adults of similar age, the fact that they're siblings is at least arguably irrelevant. There are some peripheral questions of genetics for any children coming from the union, though they are not as concerning as most people seem to think -- we allow people with birth defects to marry even though they may pass those issues onto their children, and the reality is that if there are no defective genes already present, the risk of genetic defects in sibling's children is the same as for anyone else. Then there is some question, if they were actually raised together, how it is that the natural taboo against being sexual with a sibling was overcome. But in principle, what two consenting adults -- sibling or not -- do in bed is no one's business but their own.

Almost any other configuration of incest is morally repugnant because either one or both parties can't give informed consent because they are too young and unformed, or, the power dynamic is wildly out of whack (e.g., I can't see any way it can be mentally healthy and savory for a [step]parent and child to have a sexual relationship, even if they're both adults). That sort of incest is incompatible with the normal functioning of the relationships and responsibilities within a family.

It was certainly the way the Ancient Egyptian royal families kept their bloodlines pure as they believed that they were descendants of the gods. See the work of Frank Ruhli et.al - American Journal of Physical Anthropology 157(3) · April 2015 

For too long now Governments, The Church and society in general have just been obsessed... absolutely obsessed with whom puts what part of their body in whom. If it's safe and mutually consensual - Butt the fuck out is my honest opinion.

Just like food... "The only people so obsessed with food are the anorexic and the morbidly obese, and so it is with the Church and sex" [nods to Stephen Fry]

I would say it is immoral largely on the grounds that incest is almost certainly exploitive. Our genes are not driven to reproduce with people that are too genetically similar. When close members are having sex I would immediately infer that at least one of them is exploiting the power dynamic within the .

It's irresponsible on the genetic level, because knowingly creating a life that is very likely to have all kinds of recessive defects expressed is not a good thing. Unless you can be tested and have all the genetic drawbacks ruled out.

On an emotional level, it's a breakdown of the normal dynamics of the brother-sister relationship. Usually there's a lot of unspoken or even only unconsciously known power dynamics (among others) in that relationship, and adding that intimate wrench to the gears is likely to cause some larger issues. If you break up with an unrelated BF/GF, they can leave and that's the end of the story. If the BF/GF relationship with a sibling goes wrong, family reunions will never be the same, and you can't easily just cut that person out of your life.

In and of itself, incest is not per se immoral, although a controversial area and as mentioned there are very valid reasons why we have instinctive and social taboos against that in most circumstances.

A good treatment of some these issues turns up in the novels of Robert Heinlein, such as "Stranger in a Strange Land" and "Time Enough for Love".

What is always a problem in any relationship, especially a sexual one, is any large power imbalance and any lack of consent or incapacity of one party to validly give consent. Then it's abusive, and in many cases incestual relationships would also cross that line; but that is where the immorality stems from, and it is exactly the same issue as for example a teacher forming a sexual relationship with a current student of theirs, even if above the local age of consent.

One of the things that seems to be dramatically changing in recent decades is how we approach relationships and how we think about morality around that. A huge change in living memory is the development of reliable contraception, which I believe has demonstrably resulted in a massive drop in infanticide and child abandonment compared to past centuries, and also (for the most part) effective and safe treatments for most STDs which again is a game-changer.

The other perhaps less visible change that I believe is equally seismic in the social changes happening is ever-increasing typical lifespans for humans, which has massive implications for our patterns of relationships.

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.

Morality is not a religious concept. It is derived from an evolutionary need for human survival. Incestuous relationships are counterproductive in that they do not provide healthy genetic offspring. Incest is immoral.

A surprisingly thought-provoking question. Not least because it provokes a emotional response ("no, no that's disturbingly wrong" ) before allowing oneself a rational response "why is it wrong? Because it is immoral?" When morality is a human invention. Interesting that siblings are mentioned. Incest can also involve father and daughter/mother and son. Different reaction entirely. Even if adult and consensual surely an explotative relationship on behalf of the parent. Yet often younger women seek out a father figure and as often younger men a mother figure as a sexual partner. Latent incestuous desires or no? Egyptian pharoahs regulary married their sisters. Greek myths punished incest between parent and child (usually because it involved patricide to get to the mother). The Doors , The End ... Jim Morrison sings " Father i want to kill you, Mother i want to ... (screams). Culturally taboo to protect the species seems to be the way to go, with regard to siblings at least. (I am discounting child abuse in all of this and only talking of consensual adults). I believe in compassionate understanding rather than retribution and punishment in such cases. As i said, thought-provoking ...

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.

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.

“Morals” are often simply social mores that have developed because they benefit the tribe. Incest was harmful to the tribe based on issues related to kinship and gene strength, So in that sense incest is culturally taboo, which is the linguistic equivalent of “immoral.” Limiting the question to “safe sex” does nothing to alter the morality because individual instances do not affect sweeping social perspectives. To test this, ask yourself whether you would mind reading about your activity in a certain domain in the newspaper. If you would, then that activity is culturally taboo or “immoral.”

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.

It’s immoral because familial love lacks a sexual desire, which is probably a genetic program to prevent the deformities and abnormally caused by narrow gene pools. It’s programmed into us as gross, or damaging as mechanism to protect the species, as well as individuals. The disgust reflex is to protect us from threat to health. This person asked you this question because his/her assumption is that Atheists are immoral. Basically, ask her whether it's delicious or healthy to eat shit. Because that is the kind of question they are posing you, and it’s fucking stupid. Of course there is a phenomenon where siblings separated at a young age meet by chance later in life and fall in love and have a sexual attraction. That is understandable and I think very psychologically difficult.

I can admit that I don't have enough of an education to be able to address this dilema.

Slightly off topic:

Anyone know how the Lanisters make a king sized bed?

They push together two twins!

I have scanned the comments and present this scenario: You have met and married someone in your town or city and you've had a wonderful marriage and raised several beautiful children and you decide to go to one of those DNA site to discover your family history and find out that the person that you've been with is actually a sibling. Now what? They believed that they were morally right. The unspoken truth in society is that many people's first sexual experiences have been with family members. We shouldn't have a problem with it if it's done respectfully. If it leaves a bad taste in your mouth then go wash your mouth with atheist brand mouthwash that's made with plenty of ration. lol. Does morality equal respect?

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”

it's just a cultural issue. just like not boning 15yo's is a cultural thing.