The evolution of sex is one of the major unsolved problems of biology. Even those with enough hubris to publish on the topic often freely admit that they have little idea of how sex originated or is maintained. It is enough to give heart to creationists. Indeed, the persistence of sex is one of the fundamental mysteries in evolutionary biology today. Discover , February 1984, p. 24.
Sex is something of an embarrassment to evolutionary biologists.
Textbooks understandably skirt the issue, keeping it a closely guarded secret. We do not have a compelling answer to the question. From an evolutionary viewpoint the sex differentiation is impossible to understand, as well as the structural sexual differences between the systematic categories which are sometimes immense. Nilsson, p. 1225. One idea those attending the sex symposium seemed to agree on is that no one knows why sex persists.