Comments do not fall on a spectrum between "absolutely sexist" and "absolutely not sexist".
Comments exist as fixed points in space, and the observers fall on a spectrum of "absolutely going to call the comment sexist" and "absolutely going to call the comment not sexist".
The men who are "clamming up" are judging the audience of their comments and deciding that the potential costs of honesty are just too high compared to the benefits.
They know that they will no longer get the benefit of the doubt and that they will be convicted without trial. If the punishment for traffic infringements was death and you were immediately judged, convicted and executed by the police officer on the scene, there will be a large number of people who would simply stop driving.
