To me it's all about timing.
Abortion is acceptable prior to development of the central nervous system sufficient for pain awareness.
To point: If sperm and egg don't unite, they are just some of the billions of cells that die inside us everyday. Prior to significant development of the nervous system, the zygote or fetus feels no more pain than the multitude of other cells dying everyday. If sperm and egg do unite, it's not magic, it's not a miracle, it's simply the continuation of a very complicated biological process.