Well they can try, but the evidence is stacked against them. It is well documented that human incest produces birth defects. This in effect, is directly causing harm to a third party.<quoted text>
There is a case right now of a father and a daughter who were separated when she was young. They met, formed a sexual relationship, then found out they were related.
They are trying to prove that there is a biological attraction there because they are blood related, when in most cases, if they had been able to stay together, forms a mental block that tells the brain that person is off limits. Kinda like if your sister was hot, you'd still think she was gross. But if you were separated, you wouldn't see it that way. In other words, they are trying to prove that this is natural, and biologically sound.
Evidence shows pheromones repel us from our relatives.