To me, it’s that many of these activists are worried the ship of social justice will fall apart if you pick at it. Open yourself to a nuanced view and you risk becoming one of those bigots that justifies their hatreds under the guise of “free speeeeeeech.”
The truth is that it’s possible to campaign for social justice while acknowledging that some moves go too far. Protests are fine; violent protests are not. Be inclusive; don’t roast someone because they don’t know the difference between bisexual and pansexual. If you show that your movement is complex, thoughtful and considered… well, you won’t persuade diehard Milo and Trump worshippers, but you will convince moderates that you have the right idea.