Trouble is, the Roman Catholic church sends out its missionaries, and these missionaries spread the word about no contraception, and the converts take it on, and spread the word to non-converts they know and so it spreads outside the faith. Some of the people in the less educated areas will take info like this as (for want of a better word) gospel if it comes from people (missionaries) that they consider to be better educated than they are.
Incidentally, interesting to see a member of the Higher Echelons of the Church here, the PoppaGator of the Faith....