You're defeating your own argument. Assumption doesn't mean it's the rule or it's true. There's no census on 4chan and there's no reason to assume that the assumption is correct.
I could argue against your views by citing the 4chan conventions which had quite a number of women and minorities. Obviously the largest group was 18-25 white males but this was by no means a significant majority.
I can also cite my own experiences in 4chan IRC channels where a lot of our members were located outside of Europe and America.
Or how about doing a census on /cgl/? It's well known among experienced users that the vast majority of users there are women.
You're not stating a fact.
Whether the assumption is correct or not is immaterial to the point I am making.
> I could argue against your views by citing the 4chan conventions which had quite a number of women and minorities.
That doesn't go against what I'm saying. Re-read my comment.
On the other hand, you also get a white male middle-class privilege by default, don't you?
Yes, but these are assumed to be male, white, American (or European), not poverty and right-wing ones, respectively. If you intentionally or unintentionally reveal that you are not one of these, you're in trouble.