I think that arguably in the case of a parent/child marriage, there is a huge imbalance of power and there's a good chance that the parent may be abusive/exerting undue power over the child. In that case, I can see why there would be a safeguard to protect people from that, even adults.
Actually, I think the genetics argument is valid -- when it comes down to it, aren't marriages between close family members prohibited because of genetics -- because of the likelihood that a child born from such a relationship would be more likely to have serious physical problems? Personally, I don't give a rat's ass if brothers and sisters want to marry each other. It creeps me out, but who am I to say what they can or can't do?
ETA: James, I don't think it's possible for a father and daughter to be the "same legal age" -- did he impregnate someone while he was an embryo?
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