I think the reason that a lot of people accuse Ruben and others like him trying to protect kids of being being an alt-right/Nazi extremist is because they, at least according to this thread, mostly identify with groups that have high concentrations of predators, like porn addicts, trans, and furries. Rather than admit these subcultures have particular problems with pedophilia, and that that's the reason Ruben wants to ban so many people from these groups, it's easier to just entertain a victim complex and think "he's not actually trying to protect kids, he's just an evil bigot chud trying to oppress us!" They don't necessarily have to be predators themselves, but nonetheless identify with them enough to have this reaction.
Despite its stupidity, calling him a Nazi in particular actually has some precedent beyond chronically-online communities and children's games, if you get into political history. There were various weirdo intellectuals in the 20th century that said that sexual morality was fascist, or even that it was the psychological root of fascism, like Wilhelm Reich and Herbert Marcuse. Despite the academic and intellectual dress-up I think it's essentially the same line of thought as a lot of Ruben's detractors nowadays: "I want to be a sexual deviant, feel oppressed when people tell me not to, and I'll call them Nazis in order to get what I want."
