When, under the title “Sexism is Hard to Explain”, we read of a door opening example followed by an explanation, it’s natural to interpret that as an example of sexism.
I read the first few comments; too many to read them all. Many people described similar small acts also as sexist (or racist) acts deeply resented by the woman (or black person).
The takeaway is that no man can possibly predict which small act of human interaction is going to be deeply resented by the female recipient as that of a sexist pig. And there will rarely be any outward show of that resentment, so a man never knows in what contempt the woman now holds him.
Such is life in the new world of micro-aggressions.