With a POD of your choice make polyandry in western society (that would be Europe and its offsho0ts) socially acceptable by majority. Such practice can be rejected and condemned by parts of society (could be religious groups, chauvanists ....) but majority accepts it or at least not opposes it.
Bonus points if having more than one husband is seen as status symbol and there is competition about which is better one (looks, education ....). Status of children born from such relations, both in they eyes of law and society, is of your choice.