It’s not “mainstream liberalism” that’s acting like that any more than it’s “mainstream conservatism” that’s waving nazi flags over Florida highways and calling every Republican that doesn’t grovel to Trump a RINO.
Also tolerance for people and tolerance for what they are recognizing as intolerance are not the same thing. If person A says I’m “X”, and person B says “being X is an abomination” or “you’re not X. I’m telling you you’re Y” or something along those lines, then person B is intolerant of person A. It’s a perfectly reasonable position to say “we should be accepting of X, and Y, and Z and all the other letters, and fuck anyone like person B who says otherwise”. Person B can argue that what they said or meant was misunderstood and wasn’t intolerance. But if they meant what they said, they don’t get to demand that just because someone else is tolerant of people of different physicalities, cultures, religions, races, ethnicities, nationalities, orientations, identities, etc. that they must tolerate their intolerance too.
And if you are intolerant of people who are “X” and don’t want to be called out on it, then you shouldn’t express that intolerance or act on it. If you do, you should do so under the understanding that you earned the consequences for your actions.