Or.. instead of looking for a Japanese spouse in particular... (rolls eyes)
There are certain periods of time in a person's life when you can learn a language. The older you get, the harder it becomes to learn. So the best thing is to start young when your mind hasn't slowed down, or take an intensive language course when your older.
In theory, it's not possible by immersion (according to sociologists and psychologists) because the mind is set up to learn language at certain stages in life. Once that time has passed, it's near impossible, at best difficult. Of course that time is different for everyone.
SacredBlue made a good point of mentioning "I strongly believe it won't be possible to strive in an environment where one doesn't even know the grammar of the native tongue being spoken." When I was taking Spanish from 1st grade to 7th and stopped all together, I noticed that after knowing all the declensions, tenses, etc. it was only a matter of grammar. Ergo, learn you grammar! ども。