I understand this. It's not the first time I've run into it, but in all honesty, it is the first time it's been used on me and not just by me. Feel free to correct me if I'm wrong, but you feel that the conclusion, even if you agree with it, is not valid if it is arrived at by a faulty premise or logic step. Is that correct? Assuming it is, allow me to applaud you. I too feel.that the reasoning must be as valid as the conclusion.
It's not the providing of that sustenance, but the source of it. Keep this in mind, at no point do we require a woman who has given birth to provide breast milk for the infant. And for the sake of the argument let's assume she is capable. She can't be legally compelled to provide that breast milk even if failure to do so would result in the infant's death. We this principal extended into other areas as well. No one can be legally compelled to provide blood or organs, even if in that denial, another person dies. Even in the promise of an organ or blood, up until they are removed from the body, a person has the right to stop it. I could agree to provide a pint of blood, needed immediately to save a person's life. At a half pint I can have that process halted, even if it means the other person dies because they didn't get the full pint. There is no difference in this principle than a woman wanting a pregnancy to end.
But that's not the contention, at least not by me and many others, although I will acknowledge others have used that. Also keep in mind that an argument of incomplete development might not be a justification in and of itself, but only a counter argument. Such as the prolife argument of causing pain to the ZEF is countered by pointing out that for a majority of abortions, the systems for feeling such pain is not yet developed.
The contention is that the ZEF has no rights to the bodily resources of the woman, and thus she can have it removed at any time. If the process can be done without terminating the ZEF AND at no additional physical trauma to the woman, there is no principal that says she has the right to have it terminated, only removed.