The term "parasite" in this context in the post you refer to comes straight from the Libertarian Rothbard, and his opinion. Apparently, it is not the only possible Libertarian view on the subject.
While I fully support the idea of the mother as the sole decider on abortion, I'm not sure the Libertarian reasoning is the best way of arriving at that conclusion, to put it politely. How convoluted Libertarian morals can become is also nicely illustrated by this post and my comments there.