Sexual Darwinism

It struck me as I was reading up on Darwinism for a paper on the 19th century (which I should be writing right now, instead of this) that what if there is a natural limitaion built in for every species? A limitation manifested through a change in sexual habit.

I know this may sound controversial (both ways of the HBT-debate), but try to ignore that and think about it for a second. The human race is at a point in the history of the world where we are soon to reach population numbers higher than what this planet can support.
We've come this far because our brain capacity has evolved further than any other animal. The same brain capacity has allowed us to see ourselves as individuals, and has also given us freedom to do whatever we like with our lives, also something no other animal can do.

This includes the freedom to choose to masturbate, to have sex with a member of a different species, tohave sex with somebody of the same sex or simply not to have sex. Obviously everybody won't turn into homosexuals, but it is out there as a choice and perhaps it's that very choice that is nature's last protection against over-population, the sexual darwinism.