Except, the fetus did not ASK to be put there. A decision made by the host resulted in the arrival of the fetus inside it. Even if precautions were taken to prevent the pregnancy, the act of intercourse inherently carries the risk of pregnancy, and that is a decision the mother makes in the case of consensual sexual intercourse.
It would be like consenting to sex, waiting until penetration, and then claiming you revoked permission, the other partner could not pull out fast enough, and you want them arrested for rape. Assuming, of course that your partner had the choice to REMAIN inside you and live, or pull out and die.
I'm totally pro-choice - by the way. I'll leave it up to you to decide if you want to bring a life into the world or not... I just think this argument isn't as compelling as it seems on the surface.