@Olivier5,
Life is defined as the capacity to self-reproduce. Now, there are basically two ways to do that: cloning (aka asexual multiplication) and sexual reproduction. Cloning is the easy one but it produces individuals that are exact copies of the 'parent', without much possibility for evolution.
Sexual reproduction (sex) is much more complicated. It involves two indivuduals mixing their genes randomly to produce new, totally unique individuals. This is not easy to do, and requires very sophisticated mechanisms at chromosome level.
So why go for complex sex when you can go with simple cloning? Because conditions change and thus it can advantageous to change, to
evolve.
Take an amibia. It can multiply by clonning (mitosis) very fast and does so as long as the environment is favorable. But when in crisis, ie when the amibias around start to die from one stress or another, they start to 'have sex': they mate, and produce unique new genetic individuals by sharing their genes, as a way to try and adapt to the new environment.
Biologically speaking, sex can therefore be seen as a way to accelerate evolution, by mixing genes between individuals to produce genetically new individuals, and overcome changes in the environment that are dangerous to survival of the species.