Fascist ideas are best compared to a virus that infects tolerant societies. The same way an actual virus uses your own body's mechanisms to invade and destroy, fascist ideas use the mechanisms by which oppressed or disenfranchised groups in a tolerant society claim their civic liberties, to co-opt, overtake, and destroy those liberties for everyone else.
A story I recently encountered (now lost to the vagaries of the internet, because there's been so much churn) illustrated this perfectly. Someone who was a bouncer in London bars back in the '80s and '90s was explaining why they had a blanket "no skinheads" policy, and it boiled down to this: one or two skinheads would come into a bar, buy their drinks, and just sit quietly. Fine. Then a couple more would come in and sit with them. Then a few more. And as long as they didn't get kicked out, they'd keep coming in until there were eight or ten or more, and then they would start harassing the other bar patrons, spewing racial epithets and other hate, and from there it was either a fight or a police raid. So the bar would kick them all out and refuse to serve them.
Then it would be quiet for a while.
Then one day, one or two skinheads would show up and say, "Look man, I don't want trouble, I just want a drink." And, in a tolerant society, the inclination is naturally to say, "Yeah, that's reasonable, okay."
But then a couple more will come in and sit down...
And so it goes. So it has gone through pretty much all of recorded history. This is why so many people have a blanket policy of "Always punch nazis." Because you can punch nazis when there are a few of them, or punch nazis when there are a lot of them, but eventually, you will have to punch nazis.
There are legit problems with this stance. It's too easy to just call everyone you don't like "nazi," just for starters. One reason I've always taken people to task for throwing terms like "fascist" and "nazi" around over the past decades is that the words lose their meaning. So in 2016, when we had actual, real, not theoretical fascists marching into power, people like me who objected to this were told again and again that we were overreacting. (SPOILER: We weren't.)
But the biggest legit problem with the "always punch nazis" stance (or "always ban skinheads" or whatever variant you employ), is a matter of logical consistency. On what basis can you say that it is not just all right but is in fact a moral imperative to ostracize fascism, that can't be then turned around and made into a tool of fascism?
I call this the Cake Conundrum. I.e., if I refuse to bake a nazi-flag cake, do I have a case to be upset when someone refuses to bake a cake for a gay wedding?
The answer here is yes, and I have a reason for that answer, but it is such a super-fine line of distinction that it's very hard to make work on a societal level, because it's all about context.
A gay couple getting married are, by definition, making a commitment on how they will conduct their own behavior, and live their own lives. Their choice does not threaten anyone else.
Fascism is built on the foundational idea of exalting one chosen group at the expense of all the rest. That is inherently a threat to the rights of everyone who is not a member of that group.
In a tolerant society, the former, even if it squicks you out, doesn't hurt other people, and therefore is legit. The latter, even if it gives you perks, hurts others, and therefore is not. 
This is why the term "hate speech" was coined, to give a name to this distinction between "things that are socially divergent but don't actually do harm" and "things that actually harm others." On a societal level, whether you prefer to snuggle guys or gals is no different from whether you prefer stuffing or potatoes. But if you want to ostracize, enslave, or kill other people? That is different.
It is a weird contradiction that the argument boils down to "It's okay to ostracize ostracizers." And I don't think it will ever stop seeming weird. But I don't see how you can have a functional and still free society without it.
Once again, it's like the virus model. Being intolerant of intolerance is the vaccination that prevents the virus of fascism from being able to invade and destroy.
(Note: Comments closed because I have better things to do than listen to the usual trolls coming out of the woodwork and trying to distract, deflect, and distort. If you want to have an honest discussion on the topic, I can be reached through private channels.)
 Yes, there really are such things, as bonkers as it sounds. Forgetting for a moment the extreme cognitive dissonance of "I love fluffy adorable animals, and genocide!" I don't think I'll ever understand how "I've been ostracized and it feels bad..." translates into "And now I want to do it to everyone else!"
 There are people, and mind you I know 'em, who are like, "Eh, fascists aren't that bad..." These people are generally white, straight, and (almost always) male. In other words, "it's not a problem for me, therefore it's not a problem." This is the definition of privilege, and it really pisses them off when you say so. But it really pisses me off when I see it, so.
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