There are a
number of theories about the evolution of
sex. Biologists have given this some thought for several reasons, one of the most important of which is that believe it or not
sex has major disadvantages.
There are some species which reproduce by parthenogenesis at least some of the time, not just bacteria but even some species of reptiles. Suppose one group of lizards is all females reproducing by parthenogenesis, while another is composed of males and females. The all female group will clearly reproduce faster. Why don't sexual organisms get driven to extinction?
One factor is deleterious mutations. Suppose a mother has a bad mutation which greatly decreases her chances of reproducing. She might get lucky, but if all her children get the same mutation they will have the same problem. If there is a father and each contribute a random half of the genes to each offspring, some will flourish.
Many scientists also think variety is a factor. This seems to be confirmed by species which reproduce by themselves when times are good, but engage in sex when the going gets rough. Under these circumstances there are many more combinations of genes in existence, and a greater chance one will do well under the new circumstances.