This is a poor analogy. Pro-lifers see abortion as murder and is a moral absolute to them. On the other hand, many Libertarians feel there is some theoretical ideal they're voting for which in its totality has NO chance of ever being adopted, and which a normal person would not consider a moral issue on the order of abortion.
For instance, many people includng myself would like to see the Dept. of Education eliminated. However, if a Republican candidate who can actually win is not for eliminating it, but is still clearly differentiated from the Democrat on a number of issues, and a Libertarian who has no chance of winning wants to eliminate it, how important of a consideration is their DOE position really? It would take a really peculiar person to vote for the Libertarian based on his abolish the DOE position regardless of what difference do exist between the Dem and Republican, yet that's what Libertarians do.