Muzan of course needed a cover family. One he could live in the city with and not be suspicious when he needs to do what he needs to do there. So, he ended up marrying you. Now, he wasn't an over the top husband or anything but he wasn't horrible.

He wouldn't forget birthdays or anniversaries but he wouldn't care to remember your favorite things. You two would sleep in the same bed but not cuddle. He wasn't much on affection so if you needed comforting, he wasn't the one to go to. But he would constantly praise you and shower you with gifts. He would do anything to keep you from knowing what he truly was. So, it wasn't all bad.

Now, part of Muzan's power was being able to change his gender completely. Which he would do to trick men into getting what he wanted. It was much easier to seduce them than to talk to them. But he, of course, never let you see that side of him. Humans couldn't change every cell in their body to whatever they wanted. So Muzan knew if he let you see his female form, you would have many questions.

Normally, when he needed to be in that form, he wouldn't prepare in the house you both shared. But one day, he was running late. Cursing to herself, the woman sat down at the vanity to fix up her makeup before she left, hoping you don't come home at any moment.

Just her luck, you did come. Seeing you in the mirror, she turned her head to you, holding red lipstick in her hand. Very unamused, knowing what she would have to do now that you know.