Was Martin Luther King, Jr. Like a Nonviable Fetus?

It just occurred to me that a certain kind of objection seems applicable both to moral relativism and a popular pro-abortion position. 

In short, I'll understand moral relativism as the view that moral truths are relative to particular cultures or societies.  If your society practices capital punishment, then it's morally permissible.  If it disapproves of it, then capital punishment is morally wrong.  [A bit more precisely I'll understand moral relativism as the view (a) denying the following: there are some moral truths independent of whatever people in a given society believe or practice, and (b) affirming the following: moral truths about some action A in a society are true only because people in that culture believe that A is right or wrong or practice A.]
 
Now there are all sorts of problems with moral relativism.  If true there would be numerous absurdities which I won't go into.  Instead, I'll focus on just one which also applies to arguments involving the viability of a fetus: