Which is the most likely reason that stem cell research is controversial?

Stem cells themselves are not controversial. There are different types of stem cells. Hematopoietic stem cells are the building blocks of blood. They are found in bone marrow and umbilical cord blood. They are currently used successfully to treat a number of different diseases. These are not controversial at all. Embryonic stem cells are controversial because they are obtained from embryos. Some people believe that using embryos to produce stem cells is unethical.

The 'embryos' the anti-stem cell voice talk about contain about 75 - 200 cells, and are known as blastocytes. The cells are undifferentiated, meaning they as yet have no purpose. The Pro-Life movement refer to this collection of cells as a human being about 2-4 days after fertilization.

Blastocytes are the best source of undifferentiated stem cells, and those used would be the excess discarded after IVF. They would generally be destroyed - why not use them to save human life? For Pro-Life- destroy these 'embryos' or use them to save life? Please explain your stance!