Disclaimer: NOT ALL MEN ARE HARASSERS AND RAPISTS, NOT ALL WHITE PEOPLE ARE RACIST AND NOT ALL CIS PEOPLE ARE TRANSPHOBIC FUCKHEADS.
Now that I've stated the obvious and my brain is bleeding out of my ears from doing so, let's get on with it.
Women and not least WOC and trans* women live in a world where they can and most often do experience perpetual abuse & harassment. Rape culture is now so pervasive that you can hardly move for it. No one has ever said that all men are guilty of this, or that all men harass because that would simply not be true. None the less, many of them do and most if not all women experience it almost daily.
You may wonder why it's so dangerous to say "not all men...." well not only are you deflecting and derailing the argument, you are shutting women up. Every time a woman who is harassed says something and you counter with "not all men.." it's like you're saying, there, solved that one for you. The "not all men" argument is redundant because the men who don't commit or are accepting of abuse are not the problem. Therefore those are not the men we talk about.
When you say "not all men" I hear "maybe this man". When you say "but I'm a nice guy" I hear "he's probably not a nice guy". When you say "my experience as a man" I hear "I have no interest in listening to you". Keep those things in mind next time a woman talks about her experiences and ways in which we can help solve this and makes things better for everyone. When you say "not all men" you're saying that you don't care or trust in her experiences. When you out-right call her a liar and patronise her she thinks "not all men, but definitely this man".