I cannot remember if at an early age I stopped myself and asked the question: why are there two sexes on this planet? I believe I must simply have taken it all for granted. There were men and women, in the animal world there were males and females, even flowers have some sort of two sexes. Makes you wonder about rocks.
As a gay person myself, being a man only interested sexually in men, I could have wondered why we needed women in the first place. I certainly couldn't see a reason for them to exist in my life, I had no interest in any of them. A lesbian could think the same about men, I have met a few in my days who hated men so openly, it frightened me. It is rare though that I have met a gay man hating women, quite the contrary. I have to say that I don't really mind, I just accepted it, there were two sexes, as if it was the most natural thing in the world.
Of course everyone will tell you why there are two sexes, for reproduction, the survival of the species. Isn't that obvious? So obvious, how dare I ask the question? My answer is that it is not so obvious. Some animal species and plants self-reproduce, hermaphrodite is the term, or even intersex in humans. So why should there be a need for two sexes for reproduction purposes? And why two sexes instead of three or four?
Perhaps there is a parallel universe out there where there is only one sex, where an individual can reproduce on its own. In other parallel universes, maybe you need an orgy with at least five different sexes in order to lay a huge heavily fertilized egg right in the middle that will bring about a new monster to the world. I have to say that trying to explain biology, and why it is like this and not like that, is exactly like trying to explain the universe. Why are there planets, stars, galaxies and electrons?