>>314495 (OP)
Assuming MGE lore…
Not an issue at all since having sex with monsters literally breaks the laws of thermodynamics in that the sexual intercourse between a man and monster is self-perpetuating as the spirit energy produced by the man is, as far as is described, endlessly produced and potentially infinitely scalable in rate, and said spirit energy can then be consumed by the monster to nourish themselves, nourish the man, and have enough energy leftover to bank for themselves in their own body and give some to the demon lord.
The only limit to growth could possibly be actual physical space, but given the magical nature of the world and possible existence or development of "pocket" dimensions, this would likely cease to be an issue given enough time and energy directed towards research and development of pocket dimension production techniques and/or technologies. Especially considering the laws of thermodynamics have been broken and infinite energy discovered, basically a post scarcity society, of which if you wish to learn more on I really recommend Issac Author, of whom is embedded.
Ruling out catastrophe or unforeseen difficulties, basically the population capacity is infinite. Good job.
Assuming not MGE lore.
Really, what's the point of not using MGE lore? It's the most standardized, widespread, and consistent. Though, for academic purposes, lets. There's a reason why the human world is unlike that of MGE. Having sex all the time with any and everything isn't productive. It costs a lot of energy, is sexually unselective and therefore destructive to the gene pool, and would probably get dull very quickly. That being said, a realistic generic world setting involving monster girls, without many alterations to the generally accepted archetypes, would probably involve human males as they are now and monster girls that act exactly as human females do now: just they look like monsters. That being said, a realistic monster girl involving world's population would probably act just as population does in the real one: pretty damn well self regulated. Entertaining the possibility that the system would self-regulate itself wouldn't be unreasonable, as the population should correlate directly with resources and such, and the areas of the world were the amount of resources available is relatively stable, the population is also relatively stable. Though putting forth the proposition that monster girls, as they're generally conceived to be, and human men, as they exist now, are,of the same species as a realistic scenario is in fact quite---Unrealistic? As the point of the existence of males is that they take the genetic mutation risks (XY not XX) in exchange for higher potential reward as they can spread their genes around with far more ease as they aren't burdened with child bearing, so having a female counterpart that is even more susceptible to mutation and is even more varied than the males doesn't really fit within this model. Definitely the human male would have to drastically change to "fit" or synergize with the monster girl or else the system would probably fail from natural pressures, but that's really all semantics. Point is, if not going by MGE lore, the world of monster girls is more likely than not going to self regulate its population based off resource supply. Not an issue.
What would be interesting to consider though would be to think about a realistic monster girl involving world setting with infinite basic resources like food and water. Just think of the mouse utopia experiment but with monster girls and men. https://www.youtube.com/watch?v=0Z760XNy4VM Would the scenario be sustainable or not with the replacement of male mice with male humans and female mice with monster girls? I would like to think that it would probably devolve into unspeakable filth and decadence very quickly, but lead to eventually what could be described as world ending global total war.
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… Oh and STDs, don't forget about that shit, m8. Make sure to wrap your willy. Unrelated, why isn't there a Knight, Ilias, or Chief God flag?