Well, in my general meanderings with Battousai this morning I formulated this theory.
Why Post-Marital Sex is Birth Control.
After masturbation, you find out sex feels good.
Masturbation is usually done at ages 12+.
Therefore, finding out it is a good thing (and that sex is a good thing through this) said person seeks to "do it more" (sex and masturbation)
There were no birth control devices at the time, so very likely that sex would result in children.
Sex drive is at it's most powerul at ages 13-18.
It is socially awkward to marry under 18 (too young etc.) so people would marry at 18, when extremely powerful sex drive is wilting.
This would result in less babies, as there is no desire to have more sex.
THE END.
Close it if it's too racy, but some controversial discussion would be nice.