You know, I hear what you're saying, but I would add that, still, "society" steps in here.
Your mother and your mother-in-law are not going to walk straight out of their marriages and into a marriage with a forward-looking, or just plain reasonable, man.
Instead, they face a gamut of questioning - perceived or real - as to why they have left their marriages, followed by a period of social, financial and emotional upheaval when they leave. They then face a good , long stretch as a single, older woman. Society is not so kind to many older, single women. Not least because they often have less financial clout than single men/heterosexual couples but also because of the sexism of society. Even simple things, such as travelling around at night, without the "protection"
of a man.
My theory is that society actually makes it quite hard for women to leave. That way, women stay in marriages/partnerships and a lot of men realise that it will take some seriously bad behaviour on their part to make their female partners leave.
Having said that, I really think there is a real place for bigging up the virtues and joys of (single) autonomy versus a life lived with someone who crushes you - even if it is "only" a "little bit" of crushing. I think we all need to hear more of the joys of life outside of a crap relationship. I don't think we hear nearly enough of that, do we?
By the way, Andrea Dworkin, years ago, wrote an essay about "right-wing women" and their (supposed) mistrust of women's ability to deliver power to women as women, rather than the power (some) women achieve (sometimes) by way of men. I'm not sure how I feel about the essay - I wonder if it doesn't stereotype women, and conjecture, and blame women, rather than men - but it is an interesting hypothesis as to why some women will side with men against other women, even their own daughters.