Progressives have always been authoritarian

They just pretended to be a bit anti-establishment in the 1960s, but the authoritarian streak runs deep.  Progressives swooned over Mussolini in the 1930s, Progressives made a fetish of FDR (Google Franklin Roosevelt's looks give me a thrill), the Pulitzer Prize Committee famously refused to revoke Duranty's Pulitzer, and to this day the Academy sweeps the Gulag under the rug - preferring to focus their outrage on the crimes of America.

And so, the answer to Sonic Charmer's question is the Left has always loved Fascism, only with the Fuhrerprinzip and Nationalism played down.  But they've always loved all that "shoulder to shoulder" bit, and mobilizing society for the Big Push (War on Poverty, etc).  And if you don't fall in like, then you're in the way.  And so, they're not retarded:

One aspect of these debates that always seems to be in play here is a blurring (on the part of the left) of the difference between allowing something and mandating something, and/or between thinking something is good and thinking something should be government-subsidized. So (for example) if I don’t think the government should force insurance companies to cover contraception, in the eyes of the left I must be against contraception. This raises the question, is it just that a lot of these fascist lefties are dumb-ass retards intellectally incapable of proper reasoning?

Dude, you're in their way.  And so they're coming at you, shoulder to shoulder.  I'd do an uber post on why the Progressive Left is entirely fascist (with modifications to 1930s fascist model duly noted above), but it's already been done.

Both Parties are reliably fascist under this model.  Me, I'm voting Whig.  They're delightfully uncontaminated by late 19th Century Progressivism.