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The system we have right now in USA is fine where everything is free game except immediate directed calls to illicit activities such as violence or disorder.
Agreeable speech does not need any protections, only disagreeable speech needs it and no one should trust a government to determine what non-violent speech is acceptable.
Let the marketplace of ideas and social pressure determine what becomes mainstream or not. Force of law shouldn't be used to shut down mere ideas. >

I'll talk about anything with anybody.
The worst ideas are the ones that most need drubbing in the town square.  You don't want that stuff to fester in the dark, for the same reason you don't want it held unchallenged in the light.  Discourse is the mode of social progress.
Extinguishing people to quiet dissenting opinions is the action of a propagandist.  After that precedent is set, it doesn't matter how righteous the propagandist's message was, it will turn shortly to elitism and hate. >

I’d say it becomes fighting or no longer productive when someone decides to start making things personal. Personal attacks or jabs at the person your talking with are usually a tell tale sign that it’s no longer political discourse. 
Jabs usually mean that there’s no longer and legitimate points to share and someone is desperately grasping at straws to “win” by bullying or in a more positive light it could be a sarcastic jab showing they are surrendering. >


Something repulsive or idiotic that should be opposed strongly, but politely

Misguided sexism, racism, bigotry. ex: People that have fallen into a trap like listening to the BS that Jordan Peterson spews or someone that's repeating Fox News Talking Head Bullshit

Something unforgivable that can only be extinguished with censorship / force

Outward and unapologetic sexism, racism, or bigotry. Even in this category there's scale of measured response. As the speech slides toward inciting violence the response needs to be firmer.