Abstinence, when used as birth control, still looks like commission to me (although the concept of 'commission' isn't the most well-defined of terms).
Suppose that you would have sex in the hypothetical case that you were sterile. Then, in the real world where you aren't sterile, renouncing sex in favour of sleeping doesn't seem any less of a 'commission' than putting a latex barrier between your genitals while you have sex.
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.