You know absolutely nothing about this person and yet feel completely at liberty to issue blanket statements about their life and what they have experienced and how they 'should' feel.
White privilege is obviously real. However, that doesn't mean that a white male hasn't ever been nervous around the police or felt queer thoughts.
This is why people resent contemporary 'identity politics' and 'diversity' initiatives. Not because of the end goals themselves (that is, to mitigate racism and prejudice) but because the rhetorical methods currently employed are cloying, tone-deaf, and frankly just plain condescending. The overall point you are making is a noble and important one - to self-reflect and put yourself in other people's perspective - but I don't think this is the most effective way to convey it.
Not all white males are the same and face the same challenges. Life is very different if you are good looking, tall, athletic, outgoing vs. being short, non-athletic, nerdy and socially awkward. I was in the second group and faced a lot of discrimination, bullying and ridicule when I grew up. I probably had a lot of advantages too but it's not that simple. There are plenty of rich non-white kids who probably have it much easier than most whites.
This is almost unbelievably blind.
You don't directly know what the poster has gone through. Prefacing the statement with "as a white male" implies that you know what the poster has gone through because of his race. It is willfully blind. You can make convincing statements that white people in general in certain states at certain times face certain kinds of systemic discrimination. You cannot truthfully make the statement that the poster has not encountered serious discrimination, malice, or personal difficulties merely by knowing his race and sex.
>presumably hasn't experienced that, or they'd immediately understand
This is a Scotsman fallacy. I am certain I could find some people who have faced terrible racism that would agree with the poster's opinion.
As a white male you haven't experienced any of that, which is fine! But you should try to put yourself in the shoes of people who weren't so lucky.
EDIT: I was addressing the poster directly. As a white guy with autism, I'm well aware that white males in general can experience lifelong hardship for any number of traits. But anyone who says "I don't really understand why all these groups are around race, sexuality or gender" presumably hasn't experienced that, or they'd immediately understand why someone would want to associate with folks who've been through the same shit.
(Or maybe I'm still giving him too much credit. But I try assume ignorance rather than malice.)