What exactly is "not all men"?
"Not all men" can be the shortened version of "Not all men are like that" or NAMALT, which appeared on the chat forum eNotAlone in early 2004. It's a common rebuttal phrase usually used by men in conversations regarding gender to be excused from criticism about common or certain male behaviours.
For example, when someone comes to you with their story of a sexual assault, and you assure them not all men like that, you are invalidating their story. There's obviously no need for a defense when someone is telling their personal experience.
Then again, why's it so bad? Well, when a man interrupts a conversation about some feminist issue to remind the speaker that "not all men" do that, they actually derail, and even ignoring the fact that is actually happening.
Not only do they become the center of the conversation, but they also exclude themselves from any blame or responsibility. Pointing out individual exceptions won't help us understand or combat the behavior that are mainly committed by men. Instead of clinging to the argument that not all men are like this or like that, try to acknowledge that it exists.
Without realizing it, the act of saying "not all men" can be seen as a defense mechanism that can lead to a greater problem of institutionalized violence toward women. It's also a form of denial, and it allows you to shift the blame instead of acknowledging the actual problem and taking action to minimize this problem.
So, instead of trying to argue or seek justification to some men's action or to be excluded from certain men's behaviour and seeing every criticism or claim as an attack on all men everywhere, try to listen to another person's story without interrupting them with the "not all men" argument.
Instead of interrupting, which is considered rude, think whether or not stating that "not all men" is going to derail the actual problem. Sure, not all men do sexual harassment, not all men do sexual assault, not all men are saying "not all men", but there's enough men to make some, or maybe all women scared to go alone at night, or to even do anything.