I completely agree with you that the government enforcement of birth rates is immoral and should not be attempted. When speaking of birth control, I'm talking about mitigating the population explosion that's happening in the third world by providing, making accessible for little or no money, modern birth control methods. Providing free or low cost birth control is relatively inexpensive and would have significant increases on quality of life and on the overall state of humanity in the world. Of course it should be optional, and it doesn't need government involvement -- this an issue non profits can successfully attack.
Modern birth control methods cost money; someone's got to pay for them if the third-world people you want to stop making babies aren't going to. I believe that money can be better spent on improving the lives of those people. Better food, water, and housing will help make those people healthier, which will improve the infant mortality rate, which will naturally lead to a reduction in the birth rate. One reason poor people make a lot of babies is because many of the babies don't survive and a higher birthrate is necessary to ensure the survival of the next generation. With a lower infant mortality rate, it becomes an economic burden to have too many babies, so most families cut back to just one or two children. This is a trend that can take a couple of generations to become widespread, but large changes like this should be slow so they're not disruptive.
