And a potential life, that is enough to be protected, is ANY life that would come into existence as long as no-one interferes with the "natural" course of events after they go in motion. Condoms/pills are acceptable because they are a prior interference, no different than abstinance or coitus interuptus (pulling out, for those unfamiliar). Some methods are more effective than others, but that's beside the point.
This invokes what among the medical community is called a "slippery slope." What defines "natural?" Say a man and a woman has sex. The sperm will stay alive in the woman for almost 48 hours. If the woman was killed or beaten before those 48 hours was over, and didn't become pregnant, according to your logic there would be grounds for an additional murder case, because the assaulter interefered with the "natural course" of events which would have possibly resulted in the egg being fertilized. Before you attack me for the "only possibly" reply - consider this, not all pregnant woman give birth to live babies at the end. There's only the "possibility" that a women will carry through to the end once she becomes pregnant.
So again - determining that "life" starts when the egg is fertilized is arbitrary and nonsensical from a scientific, logical standpoint.