Carrie Jenkins in Aeon:
Women who enter voluntarily into non-monogamous relationships are a direct challenge to the idea that women are ‘naturally’ monogamous. They are socially penalised to maintain the status quo. A non-monogamous woman will be portrayed as debased and disgusting – a ‘slut’. When I have discussed my open relationships online, I have been called a ‘cum-dumpster’, a ‘degenerate herpes-infested whore’, and many other colourful names.
My internet trolls focus on sex, partly because presenting non-monogamous relationships as ‘just sex’ makes it easier to degrade them, and partly because women who violate the monogamy norm – whose sexuality is out of (someone’s) control – are a threat to an ancient feeling of entitlement over women’s sexuality and reproductive potential. In contrast, a non-monogamous man is, at least sometimes, liable to be regarded as a ‘stud’.
Apart from monogamy, the only other relationship structure that controls paternity in a similar way is patriarchal polygamy, which is stigmatised in contemporary North America, for reasons including bona fide feminism as well as racism and cultural imperialism. One effect of this is that monogamy is seen as the only fair and liberal alternative.
Actually, there are many alternatives.
More here.