What's the clear legal explanation for making incest illegal for gay siblings? I will do you a favor and tell you. Because all rights are based on moral law. Morality is a religious idea. Rights are only absolute when there is an absolute moral authority (god given), else they are given and taken by the ever changing moral ideas of the government. Many things are illegal due to morality including incest law, which has been around since before the idea of genetic abnormalities. As a society gets further from God, morality has no grounding and laws slowly allow for less and less moral actions to be tolerated. Eventually leading to a society like sodom and gomorrah. Where all morality it thrown out.