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:



If moral relativism is true then people like MLK Jr. wouldn't be moral reformers. For if people in MLK's society were (e.g.) largely racist, then racism was right and MLK was the one who was wrong--HE was immoral.  But what is even harder swallow than this is that, when MLK plays a part in reforming society, he changes from being immoral to being moral when (and only when) the society changes.  What is very strange on the moral relativist view is that MLK, while staying exactly the same in his beliefs, character, and practices, changes from being immoral to being moral. This cannot be correct.  Being a morally good person means having a good moral character--it means having a certain set of dispositions for justice, love, wisdom, temperance, fortitude, and so forth.  Moral character is intrinsic to the person. Moral relativism seems unable to account for this fact.

But the same sort of problem exists for pro-abortion proponents who think that the viability of a fetus is necessary for (a) personhood and/or (b) constitutes having a moral right to life.  First note that viability is not an intrinsic feature of a fetus--it is extrinsic.  Whether a fetus is viable is not grounded simply in the intrinsic features of the unborn, it is dependent on the availability of proper technology (among other things).  A fetus could be viable in one country due to its enhanced medical technology, but the very same fetus would not be viable in another country.  Viability is more like the weight of an object (which is determined by its mass in relation to other objects) than its mass (which is determined by its constituent matter).  But personhood is an intrinsic feature of a human being consisting of various capacities.  Whether a human is a person is not determined by what country the human happens to be in or the relevant technologies available.  So too with the moral status of the fetus. Unless something like moral relativism is true, the possession of a moral right to life is grounded in some intrinsic feature for any humans who possess such a right.  Caveat #1: Of course the legal right is not intrinsic for no legal rights are such; but here we are dealing with moral rights.  Caveat #2: Some theists might deny this and say that the worth of humans which entails rights is grounded in some extrinsic relation to God, but I have never heard anyone argue that this relation obtains only when a fetus is viable.

Ignoring for now that last caveat, MLK was like a nonviable fetus--the relevant moral features of both are intrinsic and not grounded in a relation to a societies' beliefs and practices.