The protagonist has a quite masochistic propensity.
One day, he worked up the courage and confessed that he'd like to try being dominated.
But just as he was busy worrying about Her being disgusted with him for asking such a thing, the response was motivated one.
It didn't take long before they started BDSM sessions.
After they opened their hearts, domination and submissiveness has naturally been made.
The relationship gets gradually escalated...

[From Steam]