Once upon a time, millions of years ago, before humans were created God populated Earth with Unimen. The Unimen had only one eye, ear, and nostril, one hand, one leg, and no genitals. They reproduced asexually by growing a budding on their asymmetrical head, and when the budding developed into a baby they gently detached it, and the whole tribe took care of it, for Unimen were a very social species.

Unimen were also intelligent and curious. And nothing fascinated them more than the sight of two of God's other creatures having a sexual intercourse. Sex filled them with wonder and they itched to try it, as they had great affinity for one another and liked nothing more than to be touched and hugged and tickled by their companions. All that felt frivolous, in comparison, though. They wanted more. They wanted to be merged together like the animals seemed able to do. To feel their loved ones inside their souls. To feel whole, even if only temporarily. They had no organ to enable them to experience any of that, though, so their desire went unfulfilled, and their imagination roamed unchecked.

With each passing year the curiosity of the Unimen increased and grew until their yearning for sex reached a crescendo, and they decided to do something about it. All the Unimen on Earth flocked together and built the biggest bonfire they were capable of building. Then, all at once, they knelt and prayed to God. After a short time, God, as usual, answered their prayers: "UNIMEN, WHAT IS IT YOU ASK OF ME THIS TIME?"
"Only a small boon, O great God," their elder replied.
"AND WHAT WOULD THAT BE?"
"Only that you give us the ability to copulate, O magnificent God."
"YOU ASK FOR MORE THAN YOU THINK, UNIMEN. I WILL GRANT YOUR BOON UNDER ONE CONDITION."
"Anything, O beneficent God."
"I WILL NEVER ANSWER YOUR PRAYERS AGAIN. THINK BEFORE YOU DECIDE."
Think they did, but all of them wanted nothing more than to experience sex. And so they told God. God then attached each Uniman with the one next to them and supplied the new combined creatures with sex organs. And so humans came into existence.

P.S. This is why humans have buttocks; the cleft between them is the line of attachment.