London - Birds do it, bees do it, and of course, humans too.
But exactly why we evolved to have sex has been one of the mysteries of science.
Now researchers believe they have finally found the answer.
Reproducing sexually makes us much more resistant to infection.
There are many benefits to cloning oneself, as practised by some species such as Komodo dragons and starfish.
While it may be less fun from a human perspective, it removes the need for finding a partner, as well as the potentially awkward business of courtship.
It is already known that sex allows genes to mix, helping populations to quickly evolve and adapt to changing environments.
However, for sex to beat cloning as a reproduction strategy, there must be "large-scale benefits" that make a difference to the next generation, the journal Proceedings B reports. But the theory has been difficult to test as most organisms are either wholly sexual or clonal so cannot be compared easily.
To settle the question, researchers from the University of Stirling studied waterfleas, an organism that can produce offspring both sexually and through cloning.
The researchers collected more than 6 000 waterfleas from the wild in the Scottish Borders and harvested sexually and clonally produced offspring before exposing them to a bacterial infection.
The sexually produced offspring were more than twice as resistant to the infection as the cloned offspring.
Stirling researcher Dr Stuart Auld said: "The ever-present need to evade disease can explain why sex persists in the natural world in spite of the costs.
"Cloned organisms are genetically identical to their mothers which means any parasite, or bacterial infection, that can infect the mother can infect the offspring."