Unclean does not equate to sin. Abomination is not equal to sin. However each carries a danger to the human condition by way of diseases should we eschew the warnings. Aids, syphilis, gonorhea, crabs all became household words because of filthy sexual practices. By extention it becomes a sin unto the human body. Marriage becomes the life giving route for humanity. If that isnt enough, get a second (says the prophet).