"If people want to extend their families why not foster or adopt"
It's not easy to do that, not everyone is equipped or has the right qualities for such a consideration it's an enormous undertaking. You make no allowances for the fact that not every pregnancy is planned, for umpteen reasons, contraceptive failure for one, not every unplanned pregnancy is met with the desire to terminate it, some women will wish to go ahead and have their baby. From what I read, the teenage pregnancy rate is less common than it was, you cannot assume that prospective mothers don't work after they have children and won't be tax payers, a lot are these days. That in itself is a feat, childcare is not easy, there are a whole army of grandparents out there filling in on that score. The desire to procreate is part of the human condition, you could argue it's why we are put on this earth to propagate the species, although here in the west most women are lucky enough to choose whether they want to become mothers or not, it's not the foregone conclusion it was. Certain cultures do not have that luxury, because they live under harsh patriarchal regimes where young girls are expected to start having children when they are often still children themselves. These are the demographics that tend to over population, it's not good for the women and it's not good for the children. Do you imagine such societies want western interference and a lofty finger wagging instructing them on how many children they should have per family and how having too many is impacting on the world's resources, even if that is the case. Lets pretend the problem of
over population of our average of 1.89 child/ren per couple and by extension construed as an over population rather than an under population problem of the developed world.