18+ Sekai no Hate de Aimashou

The general plot of the story is that a boy (yes, "he" is a boy) saved a man from outer space and the man turned the boy into a girl by using some kind of alien technology and force "her" to be his wife. Who's the boy? He's a normal high school student who looks like a girl and is liked by his brother. Who's the man? He's the prince of several kingdoms of the universe.

(Source: MangaHelpers)