Abortion is a strongly mindkilling topic for society in general, but it is not one for Less Wrong. According to Yvain's survey data on a 5-point scale the responses on abortion average 4.38 + 1.032, which indicates a rather strong consensus accepting it. As a contrast, the results for Social Justice are 3.15 + 1.385. This matches my intuitive sense that discussions of social justice on LW are much more mindkilling than discussions of abortion.
If a "pro-choice" essay had been under discussion, then "LessWrong is already pro-choice, of course it's not going to be a mindkilling discussion" would have been my conclusion as well. But the thesis of the essay was strongly "pro-life", and it still got a good reception, with rebuttals mostly of the form "here's what's wrong with your assumptions and numbers" rather than "go away you woman-enslaving theocrat".
It could just be that the survey questions don't distinguish between different reasons for vario...
For example, what would be inappropriately off topic to post to LessWrong discussion about?
I couldn't find an answer in the FAQ. (Perhaps it'd be worth adding one.) The closest I could find was this:
However "rationality" can be interpreted broadly enough that rational discussion of anything would count, and my experience reading LW is compatible with this interpretation being applied by posters. Indeed my experience seems to suggest that practically everything is on topic; political discussion of certain sorts is frowned upon, but not due to being off topic. People often post about things far removed from the topics of interest. And some of these topics are very broad: it seems that a lot of material about self-improvement is acceptable, for instance.