The mistress/minor wife thing exists in most cultures to varying degrees depending on the gap between rich and poor. A bloke can only afford so many wives.

But its a thing that traditionally is handled with discretion and not talked about openly in monogamous cultures. All the same, everyone knows it exists, especially in Asia. It all comes back to the losing face thing. So long as wify number one gets unlimited access to hubbys loot and is never embarrassed by hubbies extramarital excursions there is no problem with it. Effectively it doesnt exist if there is no confrontation and it doesnt cause any adverse consequences for either party.

Western culture tends to judge these things on a fundamental set of moral/social values. Asian culture tends to judge these things more on the consequences for the individual.
Thai women can be among the most jealous and possessive of any culture, yet if their mans activities have no consequences for them, they really dont want to know about it.