This isn’t NTR (cheating/cuckold) though??
It’s just someone who finds comfort through an unhealthy way which is sex and it stems from his trauma being raped, being forced to sleep with his close friend like brothers, then getting used to it. Both Pilgyun and Nanmu loves each other but hadn’t established a relationship nor commitment yet. So no NTR.
I personally don’t like it with any of the ML or MC sleep with someone else but I can’t say they are cheating either unless they were in a relationship.

This author sure loves NTR...