If someone from ideological group A dislikes something from group B and calls that out, then it's "discrimination against private views", and wrong and evil and "an ideological thing".
But if someone from group B calls out group A, then it's "documented actions" which are the cause and suddenly it's absolutely okay to lock group A out of polite society on those grounds.
Lucky for group B they have this absolutely objective, unquestionable standard for discrimination and intolerance, be it "documented actions", or whatever else is convenient at the moment. Groups A' and B' over a different issue can disagree all they want, and both of those views are acceptable in polite society, but if you're in group A then, by god, that's just not okay; that's objectively wrong!
It starts simple. "Racism is objectively wrong, and holding racist views absolutely means you should be locked out of polite society." Ohkaaay.... I can't disagree with that per se (which just goes to show how deep this problem goes). Then "sexism is objectively wrong, and holding sexist views means no CEO position for you!" Well.... I suppose that's a good thing, right? "Believing in God is objectively unacceptable." Uh, guys? "Voting Republican just shouldn't be allowed." And there you go.
I'm not saying it's a slippery slope we should never step on, because I've been brainwashed by society to absolutely agree with the first step—I've been raised to think racism is objectively wrong. Society is wearing away at my views on the second position; eventually I'll have to agree with that or go the way of the dinosaurs, and I know I will go with society, because that's what people do, whether we want it or not. But this is exactly how things like "Jews just aren't human" get started. Sure, it doesn't look like that now; surely discrimination against LGBT is something we should get rid of any way we can, right?
No. Intolerance will never achieve tolerance; discrimination will never achieve non-discrimination; punishing thoughtcrime will never achieve freethinking. To think otherwise is just, well... objectively wrong.