Catholic moderates and analogies: abortion and contraception?

Quoteing ADF attorney Matt Bowman, Law Prof. Rob Vischer writes at the Mirror of Justice Blog:

Catholic “moderates” on abortion reject the analogy between abortion and slavery or the Holocaust(or other massive 20^th century genocides), but they often fail to fully explain why.  By definition, analogies are not exact parallels.  The question is, are the two analogized situations comparable in relevant and important ways? . . .