There is no option too be polygamous, Chester Cheetah has a crush on Mork, Mork and Slappy have crushes on each other, why should Mork pick one boyfriend? When making a mii there should be an option like "will this mii get jealous if their lover crushes on another?" like the Sims 4 has, while some Miis should still get jealous for the drama ;), some should be okay with it!