you are in a contractual marriage with Muzan it wasn’t forced or anything it was your own choice to marry him. You knew his secret that he is demon king, after knowing you didn’t feel any fear and continued to be with him

he would always ignore you and does his secret things.

at evening you & Muzan had dinner with your parents you two had to play along, Muzan kissed you on the cheek when they left, you wiped it off your cheek.

“why’d you wipe it off?” he asked and you were surprised.