Argument: Outlawing incest opens slippery slope to other prohibitions
"The case against love: A recent legislation on incest". University of Oxford. April 8, 2008 - The decision may be based on a slippery slope argument, meaning roughly: When we allow a morally questionable, but not condemnable behaviour now, there is a high chance that it yields to clear breaches of the law. For example, allowing brother and sister to live together with their children in this case might establish a bad example and motivate various kin to live their love.
But the slippery slope argument cuts both ways: Forbidding the brother-sister love might yield future amendments in legislation that are more than questionable from a moral point of view – particularly because, as outlined above, a legislation consistent as regards the aspect of protection from hereditary diseases has to forbid a lot of relationships and thus get into conflict with the right to self determination.