Are you asking about the origin of sexual taboos in general? That's a very broad topic whose ultimate answers often predate the historical record – even extremely old sources like the Code of Hammurabi list a number of sexual crimes (e.g., adultery is gone over in great detail) which were probably based on contemporary social mores that had already existed. You might have better luck at /r/AskAnthropology or /r/AskSocialScience.