It's not meant to be a stereotype applied to all men, just the a thing that some men do. It happens when a man assumes, perhaps subconsciously, that the woman he is speaking to is his intellectual inferior and would surely benefit from his opinion on whatever topic without any regard to her possible expertise on the topic, or even his own lack thereof. I've rarely witnessed it myself, but know women who have had to put up with it. Stereotypeing all men as "manslainers" would be rude, but mocking the men who actually behave that way is cool with me.