Because man and woman are the natural way of nature. They can reproduce. So it became the dominate idea. Homosexuals cannot reproduce. That's why it is considered a perversion from the norm. But the media and the government are aggressively trying to change the perception of society now. It is covered in some form of media every day. The minds of the people have to be changed. It is no easy task because many see it as going against nature, religions and normal relationships between men an women. For that reason it may never be totally accepted as normal. Also, for some people, it is difficult to understand why they are not attracted to the opposite sex.