The argument is simple, really: Aborted fetuses get a one-way ticket to heaven. Abortion is murder and damns the parents to hell. Knowing that aborting a fetus will damn you while saving your unborn child for eternity makes it an eternal sacrifice. Therefore, aborting as many fetuses as possible while knowing that doing so damns you is an infinitely more moral act than raising them in a religion where they might not make the cut into heaven and be damned. What could be moral? Compare that to the weekend Jesus gave up to save only those who believe in him.
Of course this creates a moral paradox. Committing such a selfless act as choosing damnation to save your children should be enough to save such a person from said damnation. But the whole concept of heaven and hell is ridiculous and is by definition deserving of ridicule.