Ah, yes indeedy true. I guess I was thinking of abstinence. So wrong distinction. More likely, then: abortion is done to a specific embryo who is thereby prevented from being, and it's done reactively; there's no question that when you have an abortion it's about deciding to kill this particular embryo. Contraceptive use on the other hand is nonspecific and proactive; it doesn't feel like "I discard these reproductive cells which would have become a person!", it feels like exerting prudent control over your life.
Every time contraception is used, it prevents a specific multitude of "potential humans" from existing. Sure, most of them would have been prevented from existing by other factors, but contraception still actively contributes to that. It's also done reactively, in that it's a reaction to someone's desire to have sex with a lower risk of pregnancy. It may not feel the same way as abortion, but that's just because it's easier for humans to value fetuses than sperm and egg cells. Both abortion and contraception have specific and reactive components, in principle.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.