Apparently, Mackenzie Phillips had a sexual relationship with her father, John Phillips of the Mamas and the Papas.
There's been a lot of debate on whether it's possible for a relationship like that to be consensual, as stated by Ms. Phillips. While the details of Ms. Phillips relationship are weird and drug-addled, I think it could be entirely possible that the that type of relationship could be consensual.
First off, to properly address the situation, we have to set aside our initial disgust at the idea. Basically, as men, we have to set aside the unsettling idea of sex with our daughter, and as women, our father. In the paradigm of social interaction, models of parent and child are very strongly defined. This paradigm is inculcated at a young age. But why is the concept of father/daughter sex essentially disgusting?
Obviously, we understand that this is genetically wrong, because any children born would be highly likely to have developmental issues. This need to avoid malformed children seems to be genetically programmed into us and many advanced species, so that may be why it's seemingly "wrong" to us. Still, many types of social monkey groups seem to have only one sexual taboo: mother and son.
Is the reason it's disgusting because of our socially programmed idea of what defines a father and daughter? Would it be any less disgusting if a man who raised a girl from babyhood began a sexual relationship later, while was not genetically her father? Or what about a man and woman who fall in love, only to discover later that they are father and daughter? Why should the genetics make one scenario disgusting while not the other? Genetics are something we can't see, touch, taste. If the pair never have children, why is the situation at all wrong?
I admit, that I have a hard time separating the roles of father and daughter in the scenarios. Perhaps it's impossible for me to do so. But we cannot pass judgment so quickly. We should question what it is about our formulation of the concepts "father" and "daughter" that negates the possibility of sexual relations between the two. Is it evolutionary programming we cannot avoid? Is it social? I may find it disturbing, but must I?