Feminists

but a lot of the reason the default presumption is that child of divorce will reside with their mothers is that fathers aren't equal when it comes to day-to-day parenting responsibilities. The most recent data I've seen showed about 1 in 5 fathers are primary caregivers. That's low but probably much higher than it was a few decades ago.

In many states shared legal custody is now the default and where the father has been a primary caregiver he's more likely to get primary physical custody, which is good for children. Is there work to be done in the area of child custody? Sure. You won't find many feminists arguing against it.

Now the MRAs: do your own research but everything I've read suggests they're more concerned about forking over money to their exes than what's in the best interests of the children. The OP is about MRA/MRM, not men in general.