My father and his childhood friend somehow became sex partners. (Arisaka Miyuki HND-611)

My childhood friend, Miyuki Arisaka, often comes to my house to stay overnight. Tonight, the three of us slept together—my dad, me, and her—but she started seducing my dad behind my back! When I turned around, I saw the two of them having sex! Afterwards, I watched them having sex secretly at home, which made me so frustrated that I got an erection!