It's extremely rare. When the MC isekai-ed, yet the ML had other lover when they married and the MC approved that. Even cherished them. And the lover loyal to both ML&MC.
Its my first time read manga with polyamorous story in which MC not the center, and they had harmonious relationship without any jealously.
I feel complicated. And it said the author had RIP... Ah, i really don't now how it would end.