It's called polyamorous relationship. It's a thing and it is recommended if you aren't strictly monogamous. According to a study, it can make up for better relationship and overall satisfaction in couples, especially if they are struggling with quality of sexual life and miss happiness in life. It is based on concept of ignoring your selfish ego, lowering emotional dependance (so heartbreaks doesn't hurt much) and general openness for love. Basicaly makes you less human in a sense that I understand basic moral values and is a result of liberation and hypergamy. I don't get it how can be someone happy with such setting, but it's actually popular so it can't be that bad I guess. Whatever floats your boat?