We legislate morality all the time. It is the premise of democracy. We ban murder because we believe it's unethical.
I'm perfectly happy for medical boards to decide whether abortion is medically necessary. I also believe they are best suited to decide that. The issue is that activists are arguing for no such oversight. They want the mother to decide, not said medical board.
That's not the only reason; a society where murder is legal wouldn't just be unethical, is not tenable. It would quickly fall apart as every minor dispute is solved by killing those who disagree.
Unlike murder, there is no ethical cost to terminating a fetus <24 weeks, when it is not yet capable of sentience. There is no need for a medical board to intervene before this point. Allowing abortion access leads to a better society, not a worse one, in terms of crime, generational wealth, resources and attention per child, and the obvious benefits of simply not having a society filled with unwanted, unloved, and resented children. Forced-birther moralistic arguments rely on ignorance regarding fetal development and apathy regarding outcomes of both mother and child. They are not comparable, morally speaking.