It's because we're also Darwinian, like those Libertarians on the right.
How do they know we're Darwinian?
Because we support marriage equality.
I'm not kidding.
That's the only reason they cite:
This is social Darwinism of just the sort depicted by Steve Young -- destructive outcomes for the poor, inflicted by ruthless commercial forces in an unregulated environment. It is enabled by libertarianism on the cultural left.
The left very properly advocates government regulation to promote a clean environment, workplace safety and many other social goods. But when sex comes into issue, the left takes exactly the opposite position. It fiercely resists regulation, insists on laissez-faire, and ignores the human cost.
Redefining marriage (the latest left-libertarian project) only would compound our social crisis. Disassociating marriage from biological fatherhood increases the tendency of fathers not to marry. The more that we elevate choice and individual desire as trump values, the more we compromise the welfare of children.Go figure, as my brother is fond of saying.
OK, if that's the definition of Social Darwinism, put me down for it.