Why don't we approach this question from the perspective of Natural Law, that is (according to Francis Slade), "the ontological priority of ends over purposes." Before you stop reading for fear of big words, allow me to explain myself. Ends are what something is for, regardless of human intentions. Purposes exist in the mind of men. Something's end may or may not coincidence with a particular's purpose. For instance: a musician's end is to make music, but his purpose may be to make money, or to get babes, or to (perhaps) make music. If the musician takes money to be more important than making music, his intentions become entangled and his art conflicts his purpose.
Applying this though to stem cell research, we see that it is always immoral, because it puts a purpose over an end.
The end of a Fetus is always to continue growing into a fully flourishing human adult. In our stupidity, we attempt to prioritize our own purposes over natural ends. Thus, we think that a Fetus can be used to make other people well, when its end -- external to man -- is to continue growing.
In our mechanistic world, we must be careful when making moral decisions. We are not simply walking machines, but rather living beings that have ends within ourselves. Our ends are written upon our form -- human beings. Our source of change in within ourselves. When we mechanize the world, we take away from ourselves the very thing that makes us us: our humanity.
