In the post-enlightenment Europe of the 19th century the highest authority was no longer the Church. Instead it was science. Thus was born racial anti-Semitism based on two disciplines regarded as science in their day - the 'scientific study of race' and the Social Darwinism of Herbert Spencer and Ernst Haeckel.
Traditional marriage between a man and a woman has been a cornerstone of our society for generations. If we are going to change that it ought to be done by the will of the people.