1) The Constitution of Japan, in an extremely liberal reform for 1947, inadvertently banned gay marriage:
Article 24 - Marriage shall be based only on the mutual consent of both sexes and it shall be maintained through mutual cooperation with the equal rights of husband and wife as a basis.2) In Japan, adults can adopt other adults. Indeed, adults adopting other adults is the rule. (Link)
With regard to choice of spouse, property rights, inheritance, choice of domicile, divorce and other matters pertaining to marriage and the family, laws shall be enacted from the standpoint of individual dignity and the essential equality of the sexes.
Given that an individual can establish his or her own sui generis family registry (koseki), I leave it to the readers to work out what could/should/must happen next.
I confess: like much of the rest of the world, I guffawed at the speculations of actor Jeremy Irons as regards changing the definition of marriage (Link). In retrospect, the problem is not that Irons' hypothetical is too far out there: it is that it is upside-down.
So sociologists, time to get out there and start counting.