The biological reason for sexual activity is procreation. In his book The Selfish Gene, Richard Dawkins argues that the modus operandi of a living being is to pass on its genes, protecting the carriers (the children), cultivating more power, and in its own way, living for eternity through its ancestors.
Ironically, Dawkins, a devout atheist, is actually in agreement with one of the most memetic beings of our time: God.
Genesis 1:28 of the King James Bible says:
And God said to them, Be fruitful and multiply and fill the earth and subdue it and have dominion over the fish of the sea and over the birds of the heavens and over every living thing that moves on the earth.
“Be fruitful and multiply” is an extremely “sticky” meme because ...