[cynical answer]
Because the people that make the laws are more interested in adultery being legal than incest.
Here's the not-quite-so-cynical answer, and it goes back to a point you made in a different thread (and I honestly can't remember which one). It is a representative democracy, so a majority of the elected legislators want one to be legal and the other to not be. It really is as simple as that. There's no lobbying group to get incest laws repealed. The voters, overall, consider incest to be bad and laws preventing it to be good.
So, regardless of the potential harm, it comes down to the laws people want and the laws they don't (or can't) pass.
Everything you said is accurate. But I find the specific arguments for prohibition to be poorly constructed and illogical.
Speaking to the Moral Lobby: If you wish to ban incest for morality's sake, why not also ban adultery, pre-marital sex, birth control, pornography, and lust? Why not ban cursing and hateful words? Oh, and why not make going to church mandatory?
Speaking to the Its-For-The-Children Lobby: If you wish to ban incest for the sake of the sake of the unborn, then why not prohibit people with genetic disorders from procreating altogether? Why not create a license to breed?
You see, these positions are totally illogical. There is a lack of consistency so great that the entire arguments collapse.
The best response is to concede and say that there is no rational explanation. Just say: "I want to ban it cuz I don't like it." They should embrace that their civic worldview is guided by emotion, not by principles or logic.
I disagree only to the extent that it is a matter of lines. Most people in the modern era would agree that incest is on the "bad" side of the line and adultery is barely on the "ok" side of the line. It is not illogical - there are very real differences between the two activities. It is just human nature to draw the lines between them.
We may not all agree on the lines, but when more than half of us do, oftentimes there's a law about it.
And when 51 percent believe guns or certain religious affiliations are deemed immoral, there'll be a law about it, right?
Everything you said is accurate. But I find the specific arguments for prohibition to be poorly constructed and illogical.
Speaking to the Moral Lobby: If you wish to ban incest for morality's sake, why not also ban adultery, pre-marital sex, birth control, pornography, and lust? Why not ban cursing and hateful words? Oh, and why not make going to church mandatory?
Speaking to the It's-For-The-Children Lobby: If you wish to ban incest for the sake of the sake of the unborn, then why not prohibit people with genetic disorders from procreating altogether? Why not create a license to breed?
You see, these positions are totally illogical. There is a lack of consistency so great that the entire arguments collapse.
The best response is to concede and say that there is no rational explanation. Just say: "I want to ban it cuz I don't like it." They should embrace that their civic worldview is guided by emotion, not by principles or logic.
These laws are nothing more than a way for those opposed to an action to make themselves feel like they've done something while not accomplishing anything.
Some things never change.
Some things never change.
I think you got on a roll and went a bit too far with your point.
In many places, the gun laws are exactly how you describe. The religious laws, not as much, in part because religion has generally been much more protected than guns, and other stuff.
Of course, I may be completely missing your point, too. Sorry if that's the case.
ETA:
When incest is outlawed, only outlaws will have inbred children.
I think Rambo is right about a lot of this. (I just puked in my mouth). The arguments used in the gay marriage debate translate easily to other types of marriage, as bizarre as some seem. Marriage is deemed a fundamental right and most of the compelling interests the government has tried to push such as morality,etc. have all been shot down, and the same arguments can be used for incest, for example. The States can come up with all the laws they want, but strict scrutiny will be fatal in fact as they say. Factor in reproductive autonomy which is firmly entrenched in jurisprudence and the State can't even justify an overriding interest in unborn children pre-viability. So, the defect argument I think would fail. The Court has painted itself into a corner, maybe they'll back pedal a little in this next decision.