Siblings and close relatives are not allowed to marry not only because of a societal taboo, but for genetic reasons: we know that 'in-breeding' leads to a greater incidence of birth defects and genetic weaknesses being introduced into families.
Hypothetically then, if an incestuous couple had no plans to have children (or perhaps were homosexual), the only barrier in their relationship would be the societal taboo. Correct? And society can and does change...
I'm not trying to discredit your argument, I'm just curious to see what you'll say. I think it's been established here that biological children are not needed to validate a marriage.
