Uragiri no Netorase ~Kokoro made Otosarete Shimatta Saiaizuma Aina

The protagonist, Katagiri Hiroaki, weds Aina, who was only 20 years old at the time. After reaching his forties, Hiroaki is shocked by his diminishing physical and sexual vitality. A visit to the doctor revealed the problem was psychological, and he was advised to seek stimulation regularly. He and his wife tried everything, including watching AV together, but nothing worked. Soma Shoji, a former subordinate of Hiroaki's with whom he got along well, turns up in his life at some point. He learns that Shoji's sexual appetite was sated after his wife's death through cuckolding play with other couples. The info shakes Hiroaki that someone he knows is involved in such conduct, but he nonetheless shows an interest in learning more. He experiences unexplainable pleasure whenever he and his wife watch a cuckold-themed adult video he has never seen before. His desire for action leads him to ask Shoji to join him in mock cuckolding activities with Aina. Even though it's just fondling, it excites them to the point where their sexual activities are more passionate than ever. He wants more intense stimulation, so he asks his wife if they can engage in more intimate sexual activities. They did so because they had faith in Shoji, and he was eager to help out—