I think modern incest laws are also based on biology; specifically, the possible problematic outcomes of babies born to incestuous couples.<quoted text>We have the freedom to practice our religions; you don't have the right to force your radical secular extremism on us.
Where do you think laws against incest come from, if not religion?
As an FYI:
In the United States the District of Columbia and every state, except Rhode Island, have some form of codified incest prohibition. However, individual statutes vary widely. Rhode Island repealed its criminal incest statute in 1989. Ohio only targets parental figures. New Jersey does not apply any penalties when both parties are 18 years of age or older.