"You are my future husband."
Translation:Eres mi futuro marido.
Most Spanish adjectives come after the noun. However there are some that come before, such as "futuro", and even a few that can be both before and after with different meanings, such as "vieja".
There's a good article here: http://spanish.about.com/cs/grammar/a/whereadjective.htm and a dictionary with example usages of futuro is here: http://www.wordreference.com/es/en/translation.asp?spen=futuro
Just a guess! Perhaps it depends on whether the adjective defines the subject, or just adds some qualities. In other words, a white cat is a cat, therefore we say "un gato blanco", which can be expanded as "a cat, which is white". But a future husband is not a husband (yet), therefore they don't say "el esposo futuro" ("the husband, which is in the future"?)