How, exactly, it started is still a mystery. It acted like some sort of biological weapon but it had burned itself out so quickly that we didn't even isolate it before it was gone, let alone cure it. We were just lucky that the symptoms weren't fatal. At least, not to the individual.
It was the men, you see. The disease or whatever it was did something to the men. Almost overnight the men of the world were changed forever.
Now, about 59% of the world's men have absolutely no interest in sex anymore - the rise of the SN. Another 39% of the world's men are now physically repulsed and psychologically disgusted by the idea of touching women, let alone sexual contact - the S- or Sex Negative.
This leaves about 2% of the world's men, the only ones still interested and capable of sexually performing. The S+, the Pozzies, or the Sex Positives.
The world of gender politics has changed forever.
OOC: Those of you who've RPed with me before know that this is basically a less extreme version of my GPBL idea. A world to be in where only about 2% of the world's men still want to have sex and propagate the species, so hijinks will undoubtably ensue.