In my experience it's really an uphill battle to prove that something doesn't exist, which is what you're claiming in your first sentence.
Look into these commonly held ideas: white privilege, intersectionality theory, identity politics, and the oppressed vs. oppressor worldview. All of these commonly espoused ideas rely to some degree on the idea that it's not okay to be white.
No, they really do not. They rely on the connection between racism and power. Neither of which imply that it's not OK to be white, black, green or blue. What they imply is that it's not OK to use racism to impose power. And I doubt you'd find many who would seriously disagree with this ... and those few who would will be clearly avowed racists.
Someone gave an example above about a "fat lesbian" college teacher (already the prejudice is apparent) stating that all white people are racists. And although I have little doubt this is what some people HEARD her say, I also have little doubt that they have no idea what she was actually saying. And what she was actually saying was that if you are white-skinned and you live in a white-skinned empowered society; that discriminates routinely in favor of white-skinned people, and you participate in that society and enjoy the advantages of that discrimination (as all white people do in such a society) then you are technically a racist whether you are aware of it or not.
Which is quite reasonably true. And I suspect the teacher stated it in a way meant to catch the class by surprise, and incredulous, hoping to make them face the reality of their own white privilege.