I guess it depends on the person's point of view regarding Marxist ends.
Violent revolution has it's place. The debate then comes down to determining it's place. You claim that if Marxism is the end goal, there can be no place for violent revolution.
Many disagree. Let's discuss the reasons why you feel there is no place for it, and let's see where that discussion goes.
For the sake of argument, let's say I reject your opinion as unfounded. Not because I support Marxism, but instead because I do not think you are qualified to make such a decree. Perhaps you can present a logical argument in favor of the conclusion that can change my rejection to an acceptance.
You realize that the structure of this sentence is such that you are saying it is American to support Marxist revolution as long as you are an idealist, right?
The "unless" sets up an exception to the statement: "It would be Un-American to support Marxist revolution".
If there is an exception to that statement, the following statement would become true about the exception, "It would
not be un-American to support Marxist revolution"
I think for one to be in favor of a Marxist revolution, they must also be in favor of the creation of a Stateless society, ultimately, and thus I would say that they would, by necessity, be opposed to the existence of the USA, but that this opposition would not be unique to the USA.
Thus, I would say that it
is an un-American principle to have, but I don't think that it being un-American is contradictory in any way.