You have a poor argument in my opinion. Using contraceptives you can reduce the risk down to near zero. Not engaging in sex you can eliminate the risk altogether. If they engaged in the activity that caused them to become pregnant voluntarily then they have been forced into nothing. The whole point of abortion is not to prevent something, I have nothing against prevention, abortion removes what would naturally become a life, or is a life already by all genetic means. Simply because you can do something isn't justification for doing that thing.
If there's one guy standing in my way of getting into med school and I can get away with killing him, doesn't mean I should have the legal right to do so. By applying to med school I assume the risk that I won't make it in.
Fundamentally, what we disagree upon is whether this unborn thing deserves to be treated as a fully realized human life. I kind of have to disregard your last argument because given my view of a conceived fetus, the analogy is completely ridiculous. There is no argument to the contrary that I can conceive (har har.) that would convince me otherwise.
And while abortion is not preventative in terms of pregnancy, that's not what I was talking about. I was talking about preventing someone from being forced to go through labor, a far more dangerous endeavor than terminating the pregnancy. No one (Well, some people are, obviously.) is arguing that people shouldn't use contraception, but the notion that having a rare and unintentional event occur while you're having protected sex must necessarily mean that you are responsible for maintaining a pregnancy that might ultimately kill you is ridiculous.
And believe it or not, I love the seatbelt analogy for this one. Driving has a poor chance of injuring you if done safely. If you're not wearing a seat belt, the chances go way up. In the rare event that you have a life-threatening, doctors wouldn't deny you care and friends wouldn't deny you sympathy if they found out that you weren't wearing a seat belt. They would try to make sure that you return to normal, not tell you that you broke your own leg and that you can make the cast yourself.
That's how I view the termination of a pregnancy, whether it's because of a lack of or failure of contraception (although I support abortion unilaterally, this is what we're really talking about). My council wouldn't be that you shouldn't drive or that you shouldn't have sex; they're both extremely rewarding endeavors when done well. There are risks. And if one of those risks becomes a reality, and there's any way to fix it, it should be done. The driver might have scars from his injuries, and the mother might suffer psychological damage (Feel free to provide data on suicide rates post-termination. It would be compelling if the chance of post-termination suicide were greater than the percent of post-childbirth suicide.), but unless those things are worse than living with poorly mended limbs or having an unwanted childbirth respectively, the treatment needs to be given out.
Perhaps a better analogy, which I don't have the energy to write out extremely coherently, is HIV. People who contract HIV are provided treatment for their illness by doctors despite the fact that having sex (sometimes protected) is one of its causes. While HIV negative patients are told to take preventative measures, once the sero-conversion happens, no good doctor would ever give a second thought to prescribing anti-viral medication to prevent the onset of AIDS, which you COULD say they should have to deal with given the fact that they chose to have sex and it led to something that is a risk of having sex. But no reasonable person would do that. Just as I don't think any reasonable person (who believes that a blastocyst or early fetus is not actually human life that merits saving.) would deny a woman an abortion.
But feel free to pick apart these analogies. They're definitely not perfect.
