Right...
Because for something to be fun, it has to have poor explanations and sexist(not in a terribly negative connotations way, but a world like this is inherently sexist, if not in the usual way) settings.
Nice philosophy there, SB. >.>
Why not just the explanation that there are (almost) equal numbers of guys as girls, it's just that due to various pop culture related phenomena, a squee factor has become associated with humans, who are low in number because monsters are monsters for a reason and often end up killing humans, whether on purpose or by accident? Possibly in addition to an anomaly/culture based factor/magical anti-boy pheromones associated with Madness City that causes it to contain an inordinate amount of females, primarily monster girls because monsters make up the majority of the population? Compound this with the idea that monsters have a slightly more female-oriented preference in gender of babies(humans is 49/51, after all, perhaps monsters are 40/60 or 35/65) born that varies from race to race, sometimes being reversed(though rarely, since such reversal is worse for reproductive purposes, and hence, survival purposes species-wide).
Science can be used to explain anything, especially in combination with magic. Explaining almost everything and leaving some stuff a mystery is far better than arm-waving it. I'm not gonna dignify it by calling the previous explanation hand-waving, because that implies more effort.