Soukan Syoukougun: Boku Dake no Mesumama

One night, a family spent time having a get-together after the father returned in a long while from his business trip.

Their son Kento got curious about voices he heard leaking from his parents' bedroom, and when he took a peek, he saw the stark naked body of his father on top of his mother's body...!

Her large, puffy breasts were being licked by his dad, and at his command, his thing that got hard again gets stuffed into her mouth, and finally, she spreads her sopping wet private parts for him. The woman he saw is not his "mother", but a dirty "female animal" whose body quivered with delight!!

Having seen her in that state, Kento wants to embrace his mother with carnal desire...!?

(Source: DMM, translated)