Is there any evolutionary reason as to why there are roughly an equal amount of males and females born?

Yes there is.

For monogamous animals (such as humans, with a slight tendency to polygamy). So ideal would be one male per female. And that is what is found, now how did it stabilize at this ratio?

It's done so by a process called stable equilibrium. If you get away from it, it will automatically push back. Imagine the following situation: Due to accidents (or maybe wars or something), there are suddenly much more women than men. Now every child that is born male from then on will be an advantage to the parent's genes, since as male in the minority that child has excellent prospects for finding (many) wifes in the future.

So slight changes that cause the birth of more males than females will be an advantage for the parents, and therefore become dominant in the gene pool. However, as soon as the equilibrium has shifted back to 50-50 and starts going to the other side (more males), being male is not that useful anymore, and it pays off more to be female -> the trend will revert and stabilize at approximately 50-50.

By the way, in polygamous species such as elephant seals, where males have harems of dozens of females, much more females are born than males. It's an economic trade-off. Making females will most certainly grant you offspring, since they will find males (the males will mate with every female;), while making males is risky since only the strongest male will get a harem. However, if he's succesful and fights his way to the top, the payoff will be huge since he gets to spread his genes through dozens of females.

There are even strategies that make 'weaker' seals to have pretty much only daughters, since a male that is not huge and dominant won't get any females. Except if he specializes on sneakily 'stealing' some from the boss (that is a strategy too), yet if he gets caught, he gets killed.

So altogether, mating behaviors of animals are pretty interesting if you ask me;)

I think the question is part of the answer. However, it should be noted that not all species of animals have an equal average number of males and females. The evolution has to do with the benefits and problems of certain traits of reproduction, and what the creature in question has had to sacrifice in order to gain said benefits.

With an unlimited food supply, it would be best to produce more offspring. It is also better for the species to be good at surviving, and for the offspring to be good at surviving. The smaller the ratio between survivability at maturity and survivability at birth, the greater the number of offspring produced will be.

A species capable of producing many offspring would have more males than females. A species capable of producing few offspring would have more females than males, assuming no vulnerability of the females or children.

Given that many human females are relatively vulnerable during pregnancy, it would make sense for the male population to be as small as possible without compromising the safety of the females (humans are social rather than purely predatory).

I'm not quoting a textbook, but this would be one (of many) valid reasons for why the population of males and females would be similar.

Actually there should be more females than males, as a male I can impregnat A TON of females, where 1 female can only have a baby every 9 months at least.

So 1 female and 100 males = 1 baby every 9 months (at the very least)

1 male and 100 females= 100 babies.

There could be. Think about it logically. If there are a lot more females, there wouldn't be enough males for all the females to have young (especially if a species is widespread). Females would die off without creating offspring and there is less of a chance for survival for that species. Too many males, and there are not enough females for all the males to procreate, and many males would die off without having any young. This could be bad for those species where the female has only 1-2 young at a time. Again, this would not be the most beneficial to the survival of the species.

Is that true?

In say China aren't there more males than females born?

Can the original poster please provide some more information?