Ideally, people would successfully use other means of contraception and they VAST majority of the time they do. But there are myriad reasons someone may come to the decision that they cannot carry a child to term. I firmly believe that being compelled to do so violates security in one’s person. There’s a long tradition in English common law preceding the US constitution that holds basic the same concept to be true. Historically common law allowed abortion up to the “quickening”, which today we might call viability even though it differs a bit.
Fundamentally I don’t think the state should be able to tell you what to do with your own body. Many important rights flow naturally from this basic premise. I think this is a principled approach rather than working backwards from an outcome.
Obviously there are alternative options to safely get the starving child in your house help. You can easily call the police, CPS, etc to take care of it with no danger to your own body.
You cannot do the same with a fetus inside your body.