> I'm suggesting the exactly opposite. That by having sex and getting pregnant you don't enter any contract and you don't automatically agree to take any responsibility. You can be punished for violating terms of something you voluntarily agreed to.

I don't think this reasoning tracks. Let's say you're in a bar and get into a heated argument with another person and you decide to "take it outside", as it were. You don't have to fight this person, no one is making you, you both decided to engage in the activity. Now one of you ends up severely injured or maybe even dead, not because that was the intention but because of bad luck.

You agreed to the fight, with the known probability of significant bodily harm even though neither party desired that outcome. Do either of you have any responsibility for that outcome?

> The point is, the law should never require of you the piece of your body.

I think this is your strongest argument, because I honestly cannot think of a reason the state should be allowed to do that and the only counter argument I can muster is that there's currently no other way to bring a fetus to term, which I feel is a weak justification.