Parents' sex you shouldn't watch

Akira grew up as an only son in an ordinary family. Akira's father, Hiroshi, and mother, Natsuko, were still good friends even though Akira had grown older, and they lived together quite peacefully. One day, Akira, who had been studying until late at night, remembered something and went to his parents' bedroom to find Hiroshi and Natsuko naked on top of each other. Akira panics at the unexpected sight. The two of them are even more panicked. Akira witnesses two people who are nothing more than his father and mother having sex, and ever since then, he has been curious about their actions...