You're absolutely right!
Religion and cultural believe play a major role in overpopulation. Here in Nigeria, the Muslims literally take the words 'Populate the Earth' seriously. The men marry a minimum of two wives and there children are in tens. These people live in abject poverty, with no health care or education.
Similarly, it's a cultural believe that one's you of a certain age in Nigeria, (say late twenties or thirties) you ought to be married, and once you're married you need to have children.
The society frowns at people that chooses to do otherwise.
I believe similar cultural and religious system prevails in other developing countries with spiking population growth (Sub Saharan Africa and Southern Asia)
I totally agree with you, that religion and culture are major drivers to overpopulation.