Our genetic make-up and cultural milieu is forced on us, in both cases, by Darwinian natural selection. Most people--even creationists--have internalized this idea. But what confuses people is the question of what our genes and our culture have been selected for.
The goal isn't "goodness", and isn't happiness, either. It is survival. So if you do what your genes and culture incline you to do, you will maximize the probability that your genes and your culture will survive, i.e., continue in the next generation.
But is that your goal? If so, fine. You are boring and stupid. But if not, achieving your goal will, by definition, involve actions contrary to your genetic and cultural inclinations. It makes no difference what your goal is: if it is something non-identical w/ pure survival then you must reject some "natural" inclinations.
One of my goals is to enjoy life. It seems patently obvious to me that it is much easier--in my personal case--to achieve that goal if I don't have children. For some people this conflict between what is "natural" and what leads to our goals is endlessly confusing.
-- Thornton
