By that token, Unions also have a 'right' to argue that managers should have total power over employers and all employment laws should be done away with, and to go to demos promoting this. But that would rather be out of keeping with their purpose as a Union.
They exist to promote their members interests in the workplace. They Unions are not there as individuals with a 'right' to opinions on what they like. They should only be speaking on issues within their remit, and certainly not opposed to their remit. Protesting against women speaking about their basic rights is definitely not in keeping with that.
(As an example, I have in a work situation spoken with Unions in relation to initiatives to get people into work and they quite rightly replied that their focus was really on people already within work, because that is who their members are and their members pay them to represent their interests as workers. So they sure understood keeping within the limits of their remit there).