Sure, and in the bible Abraham says:
“Besides, she really is my sister, the daughter of my father though not of my mother; and she became my wife.” (Gen 20:12) // end quote
However this does not mean incest is justified, it is only another sad example of the gravely twisted sense of morality in the bible.
Even if what you said about france and holland was true -which it party is, even though it’s not quite as simple as you put it- it would not mean incest can or will be an accepted part of modern society.
From BBC News:
[i]
France dropped incest from the penal code under Napoleon - 200 years ago but siblings may not marry, and in 2004 a man who was having a sexual relationship with his half-sister was refused legal paternity of his own child.
In the Netherlands meanwhile, where consensual incest is no longer prosecuted, the legal status of the child born of such a relationship is ambiguous, according to Masha Antokolskaia, an expert in family law at the Free University in Amsterdam.
Sweden is the only country in Europe which allows marriage between siblings who share a parent[/i] // end quote
In 1998 -which was the last time an incest case was brought up in court in Sweden- A 2 year old child was put in a foster home after it’s parents had married and gone public with their relationship. The parents were cousins and the courts motivation was the safety of the child.
While some countries laws may have it legal/equivocal to have a relationship with a relative there is no country that I am aware of -including Sweden, France and Holland- where incest couples are allowed to marry or have children.
Again I would like to raise the issue of the biology involved. If the negative attitude toward incest had merely been the superstition of religion inherited through socialization I would agree with you. Things such as homosexuality is to me completely natural because it is in no way harmful and the only reason people ever thought badly of it was because some old book said so. However incest is not comparable to homosexuality. Incest is against nature and the reason why is purely based on scientific fact.
From Frank B. Livingstone’s Genetics, Ecology, and the Origins of Incest and Exogamy:
Inbreeding does not directly lead to congenital birth defects per se but it leads to an increase in the frequency of homozygotes. An increase in homozygotes has diverging effects. A homozygote encoding a congenital birth defect will produce children with birth defects, but homozygotes that do not encode for congenital birth defects will decrease the number of carriers in a population. // end quote
Furthermore some geneticists have of late put the risk of producing a disabled child as high as 50%. An example of that is Human Gentics Prof. Kuntzer of the Humboldt university in Berlin. He is interviewed in this video from CNN.