A group of scientists had been trying to produce a cure for infertility by any means necessary, and they found it in an unusual plan. They had developed what they nicknamed the BWW virus, or 'birth without womb' virus. If one came into contact with it, it would modify the reproductive system subtly to allow for humans to be grown outside the womb.
The way it worked was fairly simple: after fertilization, the vagina expelled the fertilized egg rather than it implanting in the womb. The xygote then sealed itself in a hard, cocoon-like bubble while it grew to a newborn baby. In theory, this could make miscarriage and other complications of pregnancy impossible.
There was another aspect to it, though- something described as 'hyperfertility'. Whenever an affected person's vagina came in contact with a bare penis, it would instantly stimulate ovulation (without periods or the pains that come with them) to ensure fertilization unless contraceptives were used.
On top of this, it affected men in an interesting way too. It was found that if the virus spreads to a man, it mutates his genes in such a way that he changes into a woman. At first it was assumed this was a result of unprotected sex, but it soon became apparent this occurs in any situation in which the man ejaculates having come into contact with the virus.
These caveats made the scientists unsure about introducing it to the public, but one day, it spread into the water supply of a small town. People would soon be discovering its effects...