Most men are not even necessary to begin with. You need a lot of women to sustain a population but shockingly few men.
Ah! But, you see, nature, in general, abhors anything but a 1:1 gender ratio. Imagine that some event causes the gender ratio to depart from 1:1 ratio and suddenly the ratio of men to women being born is 1:2. That's not so bad, you might say, because the men can just have two wives instead of one. But it's in their offspring where things start getting interesting. You see, any individuals in generation one with any genetic predisposition towards having more sons are going to be better off than those without (or, heaven forbid, those with a predisposition to more daughters), because the males of the next generation are going to be able to mate with a ton of females but many of the females are never going to find a male. Think of it this way: you have five sons in this arrangement, your sons will have women throwing themselves at them in droves, so they can either pick a bunch of women or the "best" women out there. You have five daughters, meanwhile, and your daughters are lucky to find a man or two between them. Thus, the genes coding for "more sons than the 1:2 ratio" will start spreading, and suddenly the population will reach a 1:1 equilibrium. If the momentum is
too strong, they might overshoot and have too many males... but then the exact same processes outlined above will go into effect but reversed (females "lucky", males "competitive"), and the pendulum will swing back towards 1:1.
Over evolutionary history, this has made the 1:1 gender ratio practically sacrosanct across almost every single species (the exceptions are interesting in and of themselves, and usually have to do with strange patterns of passing along genes) that has two sexes, regardless of, say, how sexes are determined, or any of the myriad other variables that might come into play.
