Online men-dominated forums often dislike and feel personally attacked by people talking about sexual abuse/harassment done by other men. I guess they immediately imagine themselves being falsely accused of such acts, rather than being a woman that is attacked.

Right. Generally, people don't like things that can only negatively impact them.

I think groups in general feel personally attacked by things that attack people on the basis of their immutable group characteristics. Notably women dislike and feel attacked by unqualified statements like "women are crazy" or calling someone a "karen". Black people dislike and feel attacked when people talk about "ghetto trash" or "welfare queens".

Sure any individual discussion about an individual might justifiably refer to that person as "crazy" or "ghetto trash". But the nature of online spaces, and the nature of the public discourse that tends to bring these phrases and discussions into the public eye very quickly starts painting people with broad brushes.

People also feel attacked because often times discussions tend to confuse useful rhetorical devices for conveying a point with justification for a behavior that has harmful impacts on the broader group. For example, it was pretty common to here the "bowl of M&Ms where one M&M is poisoned" analogy in the height of the "Me Too" movement. It's a useful rhetorical device for explaining why someone would fell cautious about a strange man, and why they wouldn't start from a position of trust. But it's also a terrible way of generally treating men in your life, and a terrible broad philosophy for organizations and governments to follow.

And we know this rhetorical device makes bad policy and at large is harmful to innocent people because another time in recent history when that analogy was really popular was immediately after the Sept 11 attacks when talking about Muslims in general and immigrants from Muslim countries. Surely no one would find it strange that Muslims might dislike and feel personally attacked by "people talking about crimes and terrorism done by other Muslims" in the same way that many online spaces talk about "sexual abuse/harassment done by other men". Surely we wouldn't be surprised if people felt attacked or disliked an app for sharing anonymous and private information about suspicious Muslims right? Or let's say someone noticed that black people are statistically 2x as likely relative to their population to be the offender of a violent crime[1]. You'd reasonably expect people to be bothered by an app that excluded black people from signing up and was entirely about strangers providing un-verified experiences with black people under the premise of keeping people safe.

Ultimately, people are bad at statistics and really bad at understanding the degree to which a small minority of individuals can affect a large majority of people by virtue of repeat offending. So it can be true both that lots of people have completely valid awful experiences with members of a given broad group, and that members of that group feel unfairly maligned when discussion about those experiences paints with broad, unqualified strokes.

[1]: https://bjs.ojp.gov/document/cv23.pdf