They made the choice to have sex, knowing they could get pregnant. If they didn't want to get pregnant, they shouldn't have sex.I doubt I'll hit on all the things wrong with this, but I'm going to try highlighting a few.
First, the easy one: rape. While I realize that some anti-abortionists make an exception for rape, not all do, and yet they will still trot this one out. By definition, a woman does not choose to be raped. Statistically, I've seen the numbers at "1 in 6," all the way up to "1 in 4," woman have been raped or sexually assaulted in the United States. Given how many go unreported, the actual numbers are probably higher. What does this mean? It means you probably know someone who's been raped or sexually assaulted. It means I've stopped being surprised when a woman tells me she was assaulted at some point, because so many have told me that -- and I find that lack of surprise sad. It means, well, there was no choice in getting pregnant from rape.
Second, health issues. Sometimes, a woman would love to carry to term and give birth and raise the resulting child. Unfortunately, during the pregnancy she develops health issues, perhaps directly related to the pregnancy, perhaps not. Doesn't matter. What matters is that the pregnancy is complicating things to a degree that it may be healthier to terminate the pregnancy, and at times, life saving. Some treatments cannot be used while pregnant, so even if life isn't threatened, long term health may be (and certainly short-term health is). It was not the woman's choice to develop these health issues, so even if she deliberately got pregnant, shouldn't she be able to reevaluate that choice in the light of new circumstances?
Thirdly, contraception failure. We do lots of things that have a risk to them, and we don't just tell people "you made the choice, deal with it." For example, eating steak rare (increased risk of getting sick), not flossing (increased risk of cavities or gingivitis), driving while tired (increased risk of car accidents), crossing the street (increased risk of a car hitting you), not washing our hands before and after eating (increased risk of getting sick), or any number of other things that you can probably think of. We especially don't say "deal with it" to those who have taken every reasonable precaution, and still have something bad happen. We let them get treatment, and well we should if we have any empathy at all. So if properly used (or even improperly used) contraception fails, contraception that has a 99% success rater (the pill, and a bunch of others), then in what way can it be said the woman "made her choice when she had sex knowing she might get pregnant"? If you wouldn't say to the person "hey, you chose to eat that steak rare, so quit complaining about your food poisoning, and you better not be using tax-supported Medicare to pay for treatment," then you bloody well shouldn't be saying the same damn thing about getting pregnant after contraception failure. The choice to avoid pregnancy was clearly there, regardless of the sex involved.
Fourth, and for now last, ignorance. Many teens are victims of poor sex education, such that they might believe the rhythm method, one of the least reliable contraception methods available, is actually effective. Many don't realize that when the guy says "hey, I'll pull out before I finish," his pre-cum will still have sperm in it, meaning the woman can get pregnant even if he doesn't ejaculate inside her. And some, in a truly atrocious display of poor education, don't even realize sex can get someone pregnant. How does choosing sex in this case mean someone's obligated to carry a pregnancy to term that they didn't think could happen?