HESIOD DOESN'T GET IT, so I'll try to speak very slowly:
Antiwar protesters aren't Communists by definition.
But A.N.S.W.E.R. and the WWP basically are. (And of the extra-nasty Stalinist variety.)
Communists are, in my opinion, as bad as Nazis: mass murder, totalitarianism, etc. (And calling them "Marxists" instead doesn't fool anyone.)
Going to a march organized by Communists doesn't make you a Communist, any more than going to a march organized by Nazis makes you a Nazi.
But knowingly going to either one makes you icky. And calling it McCarthyism when people point that out, or point out that the Communists really are Communists, makes you either dishonest, or stupid.
(I should also note that I've tried to call attention to non-icky voices opposing the war. I think they're wrong about the war, but it's okay to be wrong. It's not okay to be in bed with Stalinists or Hitlerites.)