Unisex
In the future, sex as we know it is a thing of the past. There are no wombs in use, all is done in the lab. It begins with regular samples, collected from donors on the basis of genetic information, calculated predispositions, etc. The child is encased in a synthetic amniotic sac within a transparent artificial womb made of a bendable plastic that to the child's kicking feet approximates the inside of a matron's belly. Sounds are played in to the child; light is muted through the amber-tinted womb.
For a small while, until the wombs were created, women held the upper hand. They were necessary, the bearers of life, men were only necessary for samples and none were informed whether theirs were being used as they were taken from every male equally. This genetic upper hand translated itself almost instantly into everyday life. Pregnant women held power over our society, relegating infertile women and extraneous men to worker-bee status. Our society became a matriarchy. The essential evolutionary balance tipped and a mate was no longer required. Males began to look forward to giving samples, as they were otherwise ignored by the female race.
This triumph was short-lived. As soon as the artificial womb was made standard, the shoe was on neither foot. Neither progenitor nor parent, the populace found itself evolutionarily outpaced by itself. Furthermore, the children were not sent to live with biological parents any longer; rather they were raised in state institutions. A select number of those that applied to parent personally were paired up with another such applicant and their job was parent, under strict scrutiny. These couples were made up of any pairing of combined gender or race you can imagine, preselected by computer.
The balance of power evened quickly. Now there was no reason for distinction: nothing to prevent a woman who wanted to compete physically from taking male growth hormone and chancing facial hair and a deep voice; nothing to stop any kind of sexual adventurism; nothing ventured in such a situation, so no thrill; no reason not to, so no rush of transgression. Those that were homosexual stayed so, but the distinction was no longer distinct. The masses became more sexless, and more disinterested in sex. Without the genetic urgency, even the nerve-gatherings that stirred humans on to such ill-considered indiscretions in the past stopped firing with such urgency. Eventually humans lost the ability to reproduce, and each successive generation was to be cloned from the widely varied catalogue of samples accrued up to that point.
In the end, it was a computer error and a small enclave of aborigines left undisturbed for the purpose of external observation that resulted in the death of the old world and its subsequent repopulation.