Gosh! With all those contraceptive options available, Americans manage to make babies they didn't intend to make! How could such a thing happen!
Two words: Risk Compensation.
Risk Compensation is why people will behave more recklessly the safer they believe they are. When people perceive sexual activity as risk-free, they engage in much more of it.
Back to WSJ:
And many young people are in "the fog zone" in which their beliefs about pregnancy don't match their behaviors, according to a 2009 report by the National Campaign to End Teen and Unplanned Pregnancy. In a survey conducted by the Guttmacher Institute of 1,800 single men and women aged 18 to 29, more than 80% of both sexes said it was important to them to avoid pregnancy right now, yet 43% of those who are sexually active said they used no contraception or used it inconsistently.
See also Taking Chances: Abortion and the Decision not to Contracept, by Kristin Luker.