To think there are degenerates (disdainful) who make NTR doujins of this.
Well the author himself has like 3 NTR series (one of them still ongoing), he had that weird mini-arc in the Ashitabas which takes place in the same universe as this manga, in which mu-kun was being seduced by one of Hinata's friends from the maternity hospital and then the other husband had to punch him in the face (chp 51), then that manga ended shortly after
Then he had that super mega hyper weird manga about swinger cuckolding university couples that don't really love each other but stick together because the ones that fucks them good are the other's partner, but their social standings don't make life convenient enough to actually change partners, and they all suck
The point is, the author himself is a huge degenerate, and he probably commissioned the doujins