No one is saying that arbitrary divisions or distinctions are not useful in many applications. Doctors prescribe blood pressure medication on an arbitrary, subjective determination of blood pressure measurements and risk factors, for example.
What IS being said is that the topic deals with, literally, a matter of life or death; the distinction that is being made that human babies in case A have certain rights, but in case B they do not. So it isn’t just a matter of never accepting an arbitrary standard, it’s a matter of saying an arbitrary standard is not a good tool to use when it comes to human life and it’s possible termination.
If everyone can agree that human beings have a right to life, then it becomes a matter of prudently making sure that we don’t err when establishing any “boundary,” for the cost of that error is taking a human life, ergo: murder.
So where do we place the limit such that no error - no murder - occurs?
Viability isn’t a very good place to go for several reasons. First, two babies born prematurely, say 23 week, do not have equal chances for survival. Also, if you go down the “number of weeks along” road, you end up in the area of ethics, with another issue to resolve; if the mother carried the baby two weeks more instead of aborting it, would it then be viable? Then comes the question is this murder by deadline? Killing the baby by X date is okay, after which, it’s not. In other words, as long as you plan and carry out the extinguishing of the baby’s life before the deadline, you’re okay - but in terms of what? In terms of man-made law? God’s law? Truth? You tell me.
As has been pointed out, once you step into the swamp of viability issues, there is no getting out of the mud. If a baby at X weeks isn’t viable, then maybe an Alzheimer’s patient at X weeks isn’t viable either. Maybe coma victims after X weeks aren’t viable. Maybe blind people aren’t, deaf people aren’t, etc. etc. Where do you draw the line across which no more lines may be drawn? You can’t. Every one of those criteria were used in Germany during Hitler’s reign as justification to either terminate life or to force sterilization upon the person so their “deficient condition” would not be passed on. No, they didn’t have a full understanding of genetics… but would that have stopped them?
The horrors of what happened during that period were not achieved by one charismatic fellow showing up and delivering a lot of fiery speeches. They were achieved by the slow progression of ideas where people made decisions - arbitrary ones - about what made someone else a human being or one worthy of holding certain rights. That is the slippery slope we slide down when we start on that path.
Think I’m exaggerating the case? Then show me the Dr. Jack Kavorkian of the 19th century. Or the 17th. There isn’t one, because during that period the unanimous opinion of life was conception to natural death = human being with full rights to life. It’s only in the last century that those issues began being debated. So what is it about human existence in the 20th century that alters the definition of what a human life is or when it begins? Can you point to the single event that made us smarter, to where we can know that with any greater certainty then anyone in George Washington’s day?