When I introduced her to Grandpa, I was taken down

One day, the main character, Ryo, invites his girlfriend, Miho, into his house and introduces her to her grandfather.
While drinking the drink given by her grandfather, she gradually loses consciousness...
She ends up having a sick dream of her being cuckolded by her grandfather.

Later, when he invites Miho in again, Miho collapses.
She bought an antipyretic because she had a fever.
There were two figures groping each other on the futon.