With a POD of your choice make concubines in western society (that would be Europe and it's offshots) socially acceptable by majority. Such practice can be rejected and condemned by parts of society (could be religious groups, feminists....) but majority accepts it or at least not opposes it.
There has to clear knowledge that man is having that woman as a mistress, even though this is not neccesary spoken out loud or acknowledged by either man or woman in question.
Bonus points if concubines are seen as status symbol and there is competition about who has better one (looks, education....). Status of children born from such relations, both in they eyes of law and society, is of your choice.