Oh, now its safe to bring God into it?
First, biology- any biologist will tell you that a sperm does not have the full genetic code required to reproduce and develop into a human. Neither does a lone egg. Otherwise, women would be inadvertently commiting murder every time they have a period. It is only when a man and a women both make a specific choice to have sex and let the sperm fertilize the egg that a life is brought into existence.
If you want God involved, sperm do not have souls, but fetuses do! If it's a human soul that defines a life, which is a pretty good line in the sand to draw, then no argument that a fetus is not a human holds ANY water.