I say this because, At first, this statement seems like one which is pushing forward a belief that "Not all men do heinous crimes". That is not the problem. The sexist part is the second half of this statement. Which states that "It is always a man". Before getting into the literal meanings of this statement, Let me preface this by saying that, Most of the times a person has said this statement, they have not said it to focus on the idea of not all men being bad, but more to focus on the idea of it always being a man. That is a very blatantly sexist intention.

Now one may argue, the majority of rapes are done by men. Yes, that's true. Similarly how the majority of crimes in the US are committed by black people. Now, If I said "Not all blacks but always a black", Do you think that would be a racist statement? If you are normal, Then your answer is probably yes. And if so, you should probably agree to the fact that "Not all men but always a man" In a similar sense, is a sexist statement.

Not only does this statement give a free pass to women to do what they want and get away with it, because it clearly states "it is always a man". So there can't be women who rape right? Well, I'm sure most women and feminists understand that Women can rape. Which means most of them also understand that this statement is Factually untrue. And If you focus on my previous analogy, It is also sexist. Which is why anyone using it cannot be a respectable figure in my eyes.