How is a diverse opinion a threat to democracy?

If the opinion is that there should not be a democracy, then that is a threat to democracy.

Excluding a portion of the population from the polls

They are not excluded. They are free to vote for a party that is in line with the constitution.

its almost exclusively a phenomena specific to the left...

I don't even know what to say. In which world are the far-right, fascists and nazis known to value opposing views? Are you serious?