Our reality is a bit weird in that and few control the majority of resources. So not everyone has equal access to those resources. If they did, then children would be a blessing. We could all prosper together. As it is they become a burden on families with limited resources.
What makes a third world country? They really have little control over their resources so request aid from other countries. I think we caused a lot of it by colonizing, taken their resources and making them dependent on the US, Britain, Russia etc...
As governments begin to control all of the resources there is no longer enough to go around.
Good point, though I do think that even if we all lived in perfect communion, sharing all of our resources, there would still be a need for birth control in that resources are finite. You can equally distribute the resources for equality, but it's still not ideal if each family is only getting a potato to feed their 10 kids.
Just as an aside, I read a sci-fi book where humans discover intelligent life on some other planet. All the inhabitants live comfortably, with enough resources for all, and wars were basically an unknown concept. It was ultimately linked, not to some innate goodness of the aliens, but because they had reproductive control. They could decide whether to become pregnant or not. And so, most families only chose to have 2 or 3 kids, when they wanted them. There was no "accident" babies; each child was wanted and planned for. Now, I don't think this really would solve all problems as depicted in the story, but I do feel that there is something to it. Reproductive control is a necessary component in being able to control resource consumption.