From a blogpost entitled "I had an abortion":
It was two years ago, and I was 20 years old. I had taken a break from college after my second year, and was working full-time in a low-paying service sector job. It was about 5 months into a "friends-with-benefits" type relationship. I was on the pill. I realized that I had missed a period, and thought I should take a pregnancy test. I wasn't worried initially. I had had a couple of scares before, only to get my period a few days later. And I was a smart girl who was using birth control. Unplanned pregnancies didn't happen to girls like me.
Notice how the woman in question thought that because she was on birth control, she could never become pregnant-- and she became pregnant.
Before the advent of contraception, few women had sexual relations outside of marriage. There were correspondingly few abortions (if any).
With the advent of contraception, millions of women have sexual relations outside of marriage and a the abortion rate grows exponentially. (And I know some married women get abortions, but most women who get abortions are not married).
And when contraception fails, the only recourse is abortion if you don't want to be pregnant.