Hmmm...
Incest... that is one issue I would not take lightly upon. Actually, I've never considered the issue...
Will think about this further, and try to come up with some reasonable answer - according to my more than libertarian view - I'm way beyond more for freedom than that.
Editing my updates here:
My first inclination is to consider consensual democratic "rules" for society -- but it is kind of discriminating. However, there needs to be some minimal acceptance and trust - just like the notion of infanticide, patricide etc... Actually, I think that is the correct way of starting a deliberation on incest. If it is defined as a basic crime - then it will be such.
So why are there some things defined as "crime?"
Well, some actions far outweigh in terms of negative impact than positive impact on society and individuals.
Just like in trading - there are some things that we know from experience and insight that do simply not work.
Homosexual behaviour is not "productive" in terms of evolution, but there is more than family and children to society. Cultural contributions within science, arts, entertainment and so on are greatly in debt to many of the "counter-cultures." A free democracy means that we do not suppress these, but allow their existence and valuable contributions where they might be manifested. In short - the "worth" of a life is not only in terms of your children and earthly fortune - but also in term of other production such as through intelligent contribution. An individual is not only the flesh and blood, his thoughts - but also all the memories, legacy and influences upon others - the contributions through a lifetime - through interactions, inspiration and information contributed.
Ultimately, people like Charles Dickens or Alan Turing have had a profound impact and importance on society. And although they have no genetic impact on history - their ideas and work have left a lasting impression. Therefore they are not overall a negative impact on society - but everything has its limits ... e.g NAMBLA
http://en.wikipedia.org/wiki/North_American_Man/Boy_Love_Association