The reason, according to those ponderers, is that the genetic diversity promoted by sex is a protection against disease.
ECONOMIST: Asexual reproduction
2.
The reason for all this activity is widely believed to be that because sex produces genetic diversity, it protects a species against environmental fluctuations and, in particular, against parasites.
ECONOMIST: The longest ??headache?? ever
3.
The most important, though obvious, reason is to help us better understand sex-linked genetic defects in men after birth.