Why is birth control so important?

Mostly it was because of religion. People would only have sex to get pregnant (or that's the only reason you should back then, not that it didn't happen for pleasure of course...) but sex wasn't nearly as casual as it is today. Also Birth Control was seen as a manner of murdering children or 'obstructing god's work'. Sex wasn't talked about, it was something private, not a subject to be discussed.