I have two thoughts on this subject.
The first is Ideology.
Name me ONE part of any ideology publicly espoused by Mamono that that shames, belittle or blames men for having sexual desire (or anything really), treats men like the enemy and considers it an acceptable idea to systematically reduce the male population to 10% (or less) of the total population (whether or not that is true does not matter, it is the perception that is important).
Compered to Feminism which would be more attractive to men?
Feminists would HAVE to conduct a "purge" to remove all the elements that make men want to not associate with feminism otherwise (because Feminists state that they and only they can speak and act on the behalf of all women). women would get rid of Feminism for making women look bad.
While the (Feminist) extremists still might be around they would be countered by the (Mamono) extremists so a sort of cold war might develop.
Another point to add is if Mamono can give birth to sons or not.
If Mamono can't give birth to boys then feminists/women might have the time needed to remove the negative perceptions.
If Mamono can give birth to sons and the negative perceptions around women/feminism are not removed then I believe the (Mamono) extremists would use feminism as a reason/excuse to accelerate the monster lords agenda, using the pretense that "OUR men/boys are in danger, we must help/save them".
I hope this makes sense.