Just because you tried and succeeded in wrecking your car doesn't mean that everybody else that gets in a collision intended to the same way you did.
He shouldn't be argiing that the one is foreseeable and the other is not. So you're correct, insofar as pointing out the flaw in his logic, which he has opened himself up to in the way he argued the point. However, I would point out that there is a distinction which weakens your analogy.
In both cases there is foreseeability. However, the foreseeability differs by degree. When you drive, it is possible, though very unlikely, that you will get into an accident in a given case.
When you have sex *without birth control*, the probability of pregnancy is orders of magnitude higher than getting into a car crash. Your analogy is closer if you posit the case of the person who gets pregnant in spite of using birth control, but that is presumably the rarer case of abortions.
No one is deemed "negligent" by the mere act of driving, but one might reasonably be considered negligent for having sex without using birth control, assuming you did not want to get pregnant. The better analogy is not to the simple act of driving, but to driving negligently or recklessly. Having sex without protection ~= to driving while [intoxicated/sleepy/distracted/texting] etc.
Incidentally, I am pro-choice for the first two trimesters because I do not want the government intruding into highly personal decisions about sex and reproduction. However, I think it's a tough issue and that both sides have reasonable points when the points are argued in the most persuasive manner.
- wolf