I think that incest in real world is something we shoudn't morally accept, first and foremost to protect the people who make incest. it's not only a social matter, it has do with istinct.
everyone feels instinctively repulsion toward parents/sibling under that point of view and when that does not happen and there is a sexual intercourse between very close relatives, then istinct makes other people say: "no. stay in line".
I also think that when someone feels attracted to, for istance, his sister, it does not mean that he his different from "normal" people, it only means that he is child-like, immature: those kind of people are probably afraid of the world and subconsciously think to the their sister as the woman they can have without going outside of the house. I think they like their sister because she is the only woman they think they can have. it happens to most children with siblings but it is something subconscious and usually disappears when they grow up. I heard of teenegers feeling attracted to sisters/brothers, they are probably just a bit immature.
Maybe there are exeptions, siblings who really really love each other without being mentally disturbed or just immature, but I don't know.
YES! Let's make shit up! Let's invent facts and reasons that may have nothing to do with reality.
that being said, I'm for absolute freedom.... if two adults feel that need, although they're close relatives, then... well, do as you want, but it is morally wrong, it goes against nature.
But at least you don't want to take your inventions and fabulations and use them to make laws, that's the sole redeeming factor for your previous tripe.
There're a couple of big problems with your line of reasoning, though. Firstly, you conflate "natural" and "moral" and that's a very dangerous thing to do. The line of reasoning goes that it's "natural" for one man and one woman to have sex, therefor it's "immoral" for two men or two women to have sex. It's natural for the man to put food on the table and for the woman to take care of the children, therefor it's immoral for women to work and for men to stay at home. It's natural that the strong thrive and the weak perish, therefor it's immoral to support those who can't support themselves.
You see where this is going, right? Moral and natural have nothing to do with each other. Morality is concerned with how people should treat each other and, to a lesser degree, with how people should treat non-human beings and the world in which they live. The question of what's "natural" is, imo, a dead end. Anything that can be observed in nature is "natural" by definition, this means that "natural" takes on such a broad meaning that it's practically useless. It also means that "natural" is only concerned with how things are, not with how they can be or how they should be. A question of morality may start by looking at how things are, but the end result is always about how things should be. Those are two quite big gaps you'll have to bridge before getting from the natural to the moral, and you didn't even attempt to do so in your post.
The second problem is your use of "instinct" to derive the immorality of incest. Allow me to disagree and call you out on your bullshit. You claim that people instinctively feel repulsion towards incestual relationships, and then you stop. You don't in any way support your argument, you merely state it and expect us to accept it. What you should be doing is explain why people feeling repulsion towards something is caused by their instincts and not by their upbringing. I can't tell the difference between the two kinds of repulsion, certainly not in other people, I'm amazed that it's so simple for you. Or, more likely, you're talking out of your arse about something you only understand tentatively, if at all. By all means, go ahead and enlighten the rest of us about the different causes for the feeling of repulsion, but until you have presented a compelling argument I will continue to consider the option of any feelings of repulsion resulting from upbringing to be at least as likely as that of them resulting from instincts.
Thirdly and finally, by attempting to make the people you are talking about seem defective in some way, "child-like", "immature", "afraid of the world" and "mentally disturbed" were the terms you used, you only succeed in making yourself look like an arsehole. You don't even consider the possibility that there may be nothing wrong or even different about them, but launch directly into full "make the other group look inferior"-mode. Are you serious? All you do here is speculate about people you've never met, never even talked to. And yet you think you can dismiss them as mentally disturbed, and by implication as not to be taken seriously. But I understand you, it's the easy way to take after all. If the other person can be declared not mentally sound you save yourself a lot of time and effort by dismissing them, especially if you know the majority shares your opinion. But that's not how you get a debate, imagine I had dismissed you as intellectually inferior and left things at my first two lines. I would have saved myself the time and effort it took to type this out, but it wouldn't have been debating. Instead I decided to take you seriously and point out where the flaws are in your argument. Try doing the same sometime, I think it's a good habit top be in.