Exactly, because a man and a woman can reproduce and not two men and two women, it was/is intended for a man and woman to be married since the beginning of human existence in order to keep our species to continue to grow and not become extinct. You are correct in saying that same sex partners do not add anything to our population, so then what is the purpose of them getting married if you said and I quote, “well, the idea of a marriage between a man and woman is solely due to the fact that the pair can reproduce, making more human beings, increasing our population…?” I do not and will not agree with same sex marriage because I do not believe in doing what feels good but what is good. Before modern man, religion existed when everything that exists was being created by God but human beings are a species like no other so with the introduction of humans, came the introduction of religion to humans once there was a desire to know how we came to be. Many of the texts that you mentioned are based on the premise of greek mythology that was the main “religious” based thing that early civilizations depended on for direction. Thus, religion has always existed but in different forms. There were same sex relations because it was taught through the story of Zeus and the other people considered gods whom of which the people had nothing else to reference for guidance. So, when Christianity came along and people began to realize their wrongs, they began to turn away from them. Just like all humans-even today- people make mistakes and when they realize them more often then not they try to correct that mistake.