People learned that the F-Virus had profound impacts on the human body, but they never comprehended just how deep the genetic rewrites went.
Sure, a woman exposed to enough futa cum would being transmogrify into a busty, hung, amazon. But what about men? Since a futa's preference was... well, uninhibited, it soon became apparent that the mysteries of the virus were deeper than imagined.
Though the transformation was more gradual, even men were affected by the F-Virus' lusty ravages, becoming: the futaboi.
In time, their bodies would feminize, their voices softened, hair grew lush and long, hips plump and feminine. And their breasts swelled. Cocks grew to inhumanly large sizes. All the same changes as a woman-to-futa, except for the miraculous (and coveted) change: the growth of their very own fem-gens.