The degree to which anything is significant or life-altering is can’t be given in well-defined units, but it’s reasonable to take an unplanned pregnancy as being about as significant and life-altering as getting an injury from a car accident. Some percentage of both will be no big deal, involving an initial shock or inconvenience that soon gives way to situation-normal, but these percentages are not significant enough to keep either event from being a cause of concern. In both cases something is involved that can permanently and profoundly affect someone’s life, loved ones, and society in a way they don’t want.
But if any group of drivers suffered injuries from car accidents at the rate at which contraception fails, no one would let them drive. Rates of contraception failure per year are measured in percentages while even the most accident-prone drivers (drunken teenage boys, say) have injury rates given per capita. Whether we recognize it or not, even the most effective forms of chemical contraception leave us with a crisis of unplanned pregnancies and so can only ever be the first move in a two-stage response to unplanned pregnancy. Line up all the hard-case testimonials as you want of poor women surprised by pregnancy – so far as your only response is contraception you won’t get the numbers below crisis level.
At this point we can either allow abortion as a backstop or take steps to minimize the shock caused by having a child. The two responses might not conflict logically but they do conflict practically – any response to lessening the blow of unplanned pregnancy will probably require incentivizing the strengthening of social relationships while abortion does away with what would be the foundation of any strengthened relationship. At any rate, the contraception one uses disposes us to one option or the other. Chemicals and barriers before conception dispose to using chemicals and throwing up barriers after it (so much so that both chemicals are now delivered in the same pill) and accepting the parameters that nature works within before conception dispose one to accept them afterward.