At Accidentally in Code, catehtsen writes:
"A man left an obnoxious comment in a professional document. A more senior man noticed the comment and replied, “I too have encountered this phenomenon, both in professional settings and in online interactions.
”, signifying—I think—that he had seen it, and that he didn’t think it was funny. But he didn’t actually address the comment being there, and so it remained there for me to discover several months later."
The thing is, many men - particularly sexist ones - really only values the opinions of other men. If a man comes into a conversation with a woman assuming that he's her intellectual superior and/or starts calling her a dumb b*tch, what makes anyone think he's going to take her opinion on his sexism seriously?
It's also true that men pay a cost for not being a bystander, indeed most allies probably do. When they confront male sexists, they can be met with derision, associated with the "inferior" class ("mangina"), and lost social status (many male sexists are overly-preoccupied with establishing and maintaining social hierarchies, pontificating their pseudo-scientific theories about and categories for different levels of "males"- "alpha," "beta," "gamma," etc.).
And, of course, on a basic level, even "good men" benefit when workplaces are sexist toward women. Thus, perhaps the safest thing, for himself, a man can do in the workplace is to *sigh* about sexism, thus mildly indicating his disapproval of the practice while also not threatening the status quo.
When we hear men complain that feminists are unfairly portraying the tech industry and those who work in it as sexist when it's really "just a few bad apples" who are making things seem sexist, I think perhaps a key question I have for such men is when the most recent time was that they directly challenged sexist behavior.