Why is it not helpful to say “not all men are like that”? For lots of reasons. For one, women know this. They already know not every man is a rapist, or a murderer, or violent. They don’t need you to tell them.
Second, it’s defensive. When people are defensive, they aren’t listening to the other person; they’re busy thinking of ways to defend themselves. I watched this happen on Twitter, over and again.
Third, the people saying it aren’t furthering the conversation, they’re sidetracking it. The discussion isn’t about the men who aren’t a problem. (Though, I’ll note, it can be. I’ll get back to that.) Instead of being defensive and distracting from the topic at hand, try staying quiet for a while and actually listening to what the thousands upon thousands of women discussing this are saying.
Fourth—and this is important, so listen carefully—when a woman is walking down the street, or on a blind date, or, yes, in an elevator alone, she doesn’t know which group you’re in. You might be the potential best guy ever in the history of history, but there’s no way for her to know that. A fraction of men out there are most definitely not in that group. Which are you? Inside your head you know, but outside your head it’s impossible to.