Outlanders

Tetsuya Wakatsuki, a young photographer, documents an alien invasion that brutally annihilates the human population.
When a few mishaps lead to him being pushed up against the enemy commander, Tetsuya is knocked unconscious. Much to his surprise, he wakes up on a spaceship where the alien commander, Princess Kahm of Saint Evascuraze, reveals that she has taken a liking to him. Now she wants him to become her husband! Though hesitant to accept the proposal at first, Tetsuya soon realizes that a marriage to Kahm could potentially save the rest of humanity from certain doom. For the sake of stopping the invasion, he decides to marry the alien princess—even if doing so goes against the wishes of her father, the emperor.