https://www.dlsite.com/girls/work/=/product_id/RJ01070499.html

So......A NTR plot bearing a sacred meaning, and even the emperor tolerates his concubine partly having heart set on another man? Well played guys, well played......

It's kinda funny how both guys (and arguably the girl too) are technically getting NTR'd here. The girl (Karin) has been in love with her childhood friend for years, but has known that she's going to be the emporer's wife. When she gets married, she makes a conscious choice to put her feelings behind her and be a good wife to the emperor (Kohaku). Her childhood friend (Asagiri) is basically there to serve as hands on sex ed. He loves her back but similarly intended to keep things professional, especially since he had to have sex with other women to do his job correctly. Eventually things get to be too much and he tries to quit. The emperor brings him back and forces him to admit his feelings, which causes her to admit her feelings in an attempt to spare him the punishment. The emperor, of course, was fully aware of what was going on from the beginning and actually feels a little bad about the whole mess. As "punishment", he orders the childhood friend to serve as the bedchamber teacher until the emperor steps down. He's also prohibited from ever having penetrative sex with the girl, since the emperor does still need an heir. At the end, the emperor's thinking of eventually taking a concubine, but the sequel shows that he's decided not to back down.

Base +6