Let's start with the hypothetical that a pregnant woman is expected to die unless her fetus is aborted. In this rare, but frequent enough situation, society must choose between murdering the fetus through action and murdering the mother through inaction. Who would you choose to murder?
Abortion isn't the only topic that people make a "moral" stance about, and in some ways it's the least interesting. It hinges on an arbitrary "It's alive!" moment that's not really debatable, so much as a gut feeling.
On second thought, it does illustrate the difficulty most people have in thinking about continua rather than binary variables. Aliveness is, to me, clearly not binary. The fetus is minimally alive at conception and becomes alive at birth, but during the pregnancy is neither fully alive nor fully not alive. Similarly, an adult with brain damage, in a coma, isn't quite dead, but is also not fully alive.
I think that if we discussed specific situations, nearly everyone would wind up agreeing with the continua. It's only in the abstract that they insist on their binary labels. That's been my experience, anyway. Everyone's ethics are parsimonious, until it's their friends, family, or themselves that are in consideration.