There's a place on the paperwork where you can select "Other" and then write in the nature of the relationship. You can certainly try it if you like. What Immigration will think of it, I have no idea.
They may be more interested in your husband's planned return to Japan if you are seeking a spouse visa. The purpose of a spouse visa is to facilitate the two of you living together as husband and wife. Immigration may be reluctant to grant the visa if there will be a significant period of you living in Japan without him.