If you have the money and means not to work, then you can live there for three months, travel abroad for a day or two, go back to Japan for another three months and so on. Lucky you, you obviously have a nice healthy bank account.
Thats a little harsh to live off of a Japanese national though, just so that you can stay for a long time.
I know people who have married for pure convenience to themselves, and they find it rather miserable. More so the fact that Asian cultures tend to view marriage in a different way to westerners. Divorce is not a very seen to do thing, it can be more harmful than the actual marriage.