As someone who thought for a while they were centrist, this represents how I came to see it better than I could have put it into words.

Centrism is a desire to compromise between the two available options. There is no compromising with fascism. They might pretend to compromise, but they are really just solidifying their position for their next push. A compromise means they accomplish half of their goals and thus will have an easier time getting the rest of them than they would have before the compromise. Especially if their concessions all had nothing to do with real power, like allowing gay marriage. If they can offer the decriminalization of abortion to secure more political power, they can just consolidate that and use it to ban abortion again for everyone down the line. Their primary goal total power, everything else is secondary to that.

I see the Democrats as largely representing the status quo economically and politically with a healthy dose of social of progressivism thrown in. That social progressivism is important, but the economic and political stuff is what really needs to change to fix things. The Republicans, on the other hand, are regressive economically, politically, and socially, which was the case even before their recent descent into fascism. A compromise between those two won't do anything good, so centrism is out.