If you're looking for manga similar to Outlanders, you might like these titles. All recommendations are made by Anime-Planet users like you!
Rito was just an ordinary high school student, but that changed the day an alien princess named Lala appeared in his bath tub. Before he knows it, he's engaged to the most important woman in the universe and her jealous suitors are trying to kill him! When Lala decides to enroll at his school, Rito has to survive both attempts on his life and Lala's crazy inventions, and somehow find the time to confess to his real true love, Haruna.
They revolve and male protagonist thrown into weird situations because of a female character (alien and human). Both invovle the marriage ofe this character to the princess of an alien planet with a scary father figure. Once I started these their hard to put down.
They have the same overall theme to an extent. Both revolve around a romance, action, comedy between a princess of a galactic conquerer and an average ordinary guy.