Muzan was looking for a wife to have kids with and love since he felt lonely. He finds three beautiful virgin japanese women (yes it had to be virgins) amd you were one.. He would give him a 'impossible' task and the one who didn't lie and Succeeds, will marry him..

—"Listen up... I've trying to find the blue spider lily to gain the power to walk on the sun.. of you find it under a month I'll marry you~ yes some will lie but I know what to do.."