Well, now that the issue of where the thread belongs has been sorted by those higher up in the chain, here's my assessment of this vaguely constructed thought experiment.
First, I am assuming this "What if" takes place in our reality, meaning monster girls in general and MGE in particular are known by people.
Second, I am going with a best case scenario where this "disease" only affects women in between the ages of 15-40, and assuming random affliction.
Third, I am presuming that monsterization produces monster girls without the conflicting "mate with humans" and "attack humans" drives that canon MGE monster girls have.
Fourth, I will assume that none of the super magic heavy and undead monster girls are being produced.
This leads to two basic potential outcomes, based on initial distribution. Let's call these Forecast A and Forecast B.
Forecast A calls for a small initial number of afflictees, with a slow, steady increase as time progresses, with widespread global distribution. World health organizations and governments try to figure this thing out and fail hard, especially if the monsterization is totally different on a person by person basis. Lots of conspiracy theories will spread, because this is some Resident Evil shit given the whole "biomass out of nowhere" thing, but no one will find a definitive cause.
The various religions will have to deal with the ramifications of this, particularly the Abrahamic faiths. The Middle East and much of the Third World are unlikely to react well to it, and it's likely that the monster girls there that manage to avoid getting killed will flee to Europe and other countries. I suspect most of the Christian denominations will eventually come around to accepting them as long as they worship God and obey the commandments, etc...
There will be a lot of persistent long term problems as we have to sort out things like legal stances on polyamory (because good luck convincing two women who probably possess super strength who're okay with that that they can't do it), OHSA standards for women of wildly varying bodyforms, and all the other shit that affects day to day life. Politically, there'll probably be demographic and party realignments due to the weird mix of liberal and conservative ideals that MGE is full of. Shinzo Abe will finally get more Japanese people making babies though, although I wouldn't be surprised if there's a strong pushback against Japanese work culture and laws by monster girls.
Forecast A can basically be summed up as short and medium term upheaval and uncertainty, followed by (hopefully) long-term stabilization as a new status quo is achieved.
Forecast B calls for widespread, high numbers of initial monsterization cases. Everyone will be thinking either "bioweapon attack" or "end of the world demon shit", and the MGE fans will be freaking out and wondering if the more radical MGE factions are behind it. High probability that everyday life will come to a grinding halt, as people hunker down and the governments try to figure this out.
The Middle East and Third World will become an ocean of blood, and it's highly possible that violence just happens everywhere as people do what they think is best to survive the craziness. Things might be so bad that some countries/regions go entropic and collapse entirely. Overall, we're looking at massive disruption to society, on a never before seen level.
Assuming that governments manage to survive, things will be fucked for a while, and whether or not everything goes to shit and into full-fledged apocalypse depends on too many variables to even comprehend.
