As has been explained to you, considering the possibility that the topic should be discussed does not mean supporting it. 2 pages ago I think I pretty firmly came out and said I'm not a big supporter of the idea...
However, that said, I like to base my beliefs on logic and empiricism, not just "eww, its SICK, SICK I TELLS YA!" type statements. And I'd be more than happy to offer my opinions in public should the topic ever come up.
I'm not going to condemn to people doing what they want without a logical reason. Condemning people without a reason, simply because you find something distasteful... well, thats how bigotry starts, you know?
Now, to refresh... I don't think incestuous relationships are likely to be a good idea in most cases, because I don't think they can be genuinely consentual.
But (and its a BIG but) in the exceedingly rare cases where they ARE consentual... whats wrong with them? And no, posing the question doesn't mean I want to have sex with my parents, it means that I'm asking for a valid explanation of your position beyond ad homs.
Or you could admit that your immediate visceral reaction was purely emotional, and that you can't actually come up with a logical reason to support your initial position. You don't even need to be so angry about it.
You just don't get it, do you? Right and wrong and common decency etc are not "logic" driven. We are not talking about a maths or physics problem here. Many things in life are not approached by "logic". There are a whole host of things that are right/wrong or good/bad or wise/unwise etc that cannot be determined by "logic".
What if a person hates black people? Oh, I'll hear you say, "but that's different because you are harming others". But what if I give you the scenario of a man who hates blacks but doesn't harm them. He however refuses to associate with blacks, refuses to buy goods from any store that is owned by a black person, refuses to have any black friends etc. He hates them but doesn't harm them.
And if I said to you, "well, explain to me by logic what is wrong with someone hating blacks but does not actively do any harm against them?", you would be at a "logical" loss to explain it. Any reasonable, moral person instinctively reacts against that type of scenario. It is instinctive in decent human nature, not logical.
What about if I said what about a serial killer that can kill time and time again without getting caught? If I said to you that if you could have had 1 hr to talk to such a person, what "logic" could you use on him to stop killing?
"Oh, but he's killing people, therefore harming them". So what I ask? If he is guaranteed to get away with it time after time, there is no "logic" I could present him to stop killing. He does what he feels like and if he can get away with it, so be it. The only "logic" that counts is his, not yours. By his "logic", if he can get away with it, that's all that counts.
Thus, the only reason that you can use on such people is decency and the sense of right or wrong.
Is it "logical" for a father/mother to work like dogs to give everything for their children? Surely, logic says to do whatever is best for you. Doing what is best for someone other than you is not "logical". Yet many parents give of themselves everything so their kids can have a much better quality of life that they never had. There is no "logic" in this.
What about those people who give of themselves their whole lives for people not even their own families? What about countless people like Mother Teresa who sacrificed everything for someone else not related? Is that "logical?" How do you explain that?
The reverse situation is true, as I said before. If I knew you personally, there is no "logic" why I should not mistreat, threaten, abuse etc you. "But it harms me". As I said, if I harm you, I say "so what?" Why should I treat you right?
"Because it's the right thing to do". By who's standard? You? Society? The law? There is no logic why I should not abuse you if I want to do so. Only one thing stands in the way, and that is getting caught and reprimanded. But apart from that, there is no more "logical" argument for me to not abuse you than there is for a father to not have consensual sex with his daughter.
It is not by a sense of logic that you can explain why it's wrong for a father to have consensual sex with his daugher. It is common human decency, and it's not decent nor normal for a father to desire his own daughter for sex, nor for a daughter to desire her own father for such. It is a gross perversion of human nature and sexuality.