Heeran was the wife of that old guy, so the old guy saw it as an affair and punished nanmu by making him sleep with his “brothers”. Nanmu, thinking the old guy’s anger would lessen over time, stuck it out hoping he would eventually be allowed to marry heeran.
He only really sleeps with his 2 closest “brothers” and I think nanmu said offhandedly that it was just out of habit (don’t quote me on that) And in a later chapter nanmu meets the old man again she old man says “still doing that punishment?” So I assume nanmu is still doing it because his punishment hasn’t been lifted officially/out of “habit”

I didn't finish reading this yet. But he had to sleep with the guys just to be with Heeran? Why wasn't he with her then and now that she is dead, why is he still sleeping with those guys?