The problem here, as in other things, is the confusion between ethics and morals.

There is nothing ethically wrong with harvesting stem cells, abortion, or even cannibalism. Ethics deals with the "necessary ought" -- that which MUST be done. For example, if I were starving it would be ethical to kill and eat a pig.

Morals, on the other hand, are those things which our culture has said are "good" or "bad". To kill and eat the pig if I were Muslim or Jewish would be immoral ("sinful"), but not unethical.

The use of stem cells for medical purposes is both ethical AND moral -- both tell us that we have the duty to save life if we can. The problem only arises from where and how the cells are obtained.

If I were to force someone to have an abortion so that I could use the resultant stem cells to prolong my own life, that would be both unethical AND immoral.

If someone willingly has an abortion, I do nothing unethical to use the stem cells. Whether or not such use is immoral is totally dependent upon my own beliefs -- and then I must make the decision whether or not to prolong my life through a means which is, to me, immoral (or sinful).