You have a good point in a traditional family that doesn't view babies as parasites. However, let's not forget that many of these people are consistent: too many of them not only advocate abortion but also euthanasia of “unproductive” people. They might not call it that (though some do) but what else can it mean to have the government ration out medical care to sick people who they don't think are worth spending money to save?
Ideas have consequences, and that's especially true of evil ideas.
The one good thing about this mess is that liberals are taking care of their own problem by not reproducing. Look at the effect of reproductive rates on the growing influence of Israel's orthodox Jewish population, or the Muslim population of Europe.
Long-term, left-wing limousine liberals aren't going to win the demographic war, though I'm not at all sure traditional conservatives will, either. America in a century may be heavily Hispanic and filled with Catholics and Pentecostal Christians with names like Gomez and Gutierrez rather than Smith and Jones, but if that happens, it's our own fault.
They ARE winning: not by having lots of babies, but by indoctrinating others in public schools and the rest of the poisoned culture.