Thank you, John, for articulating all this -- for spelling it out so clearly. Your post deserves to reach a much wider audience -- I hope you're thinking about submitting it as an essay to, for example, some of the more reputable progressive news/opinion sites, such as Counterpunch and Common Dreams. It should be read by people who actually buy into that "argument."
What galls me is that it's become the slogan-of-choice among people who really should know better, but don't care because they know perfectly well that on an emotional level, the argument by reductio ad baby-killing trumps everything else.
It's unfortunate that we often counter this with arguments that come off sounding as if we're saying "If it saves only MY life..." -- which has no emotional weight (except to the speaker) and reads as merely selfish.