I strongly suspect that women who have abortions are more likely to have less self-discipline. On average, they're less likely to follow strict religious or secular disciplines. Therefore, it's reasonable to believe that they smoke more and drink more and do drugs more often than women who refuse to have premarital sex. I bet they don't even exert the discipline to exercise as regularly or eat well as women who refrain from premarital sex. And if they don't protect themselves during sex, they probably don't protect themselves around dangerous chemicals.
Hmmm… if it's not the abortions, what could be causing their increased risk of breast cancer?