Why do some pro-lifers assume that most women who get abortions are irresponsible?

"Nothing is guaranteed"? REALLY? You just said that? If you don't want a kid, don't have sex, there's no ifs ands or buts about it, or use as effective of contraceptives as possible, if you can't do that and thus decide to kill your baby due to your own ignorance, you are irresponsible.