[Gutsy]

It is currently Imperial House Law that any female of the Imperial House who marries thereby relinquishes her membership in the Imperial Family. We saw that in the former Princess Sayako's case recently. This way the size of the Imperial Family is limited as was the intention of the Occupation Authorities after WWII. The Japanese have no precedent of a "commoner" male marrying into the Imperial House.
As for a royal marriage involving a foreigner: e.g, an Imperial Prince marrying a foreign Royalty is would be a direct confrontation with the traditionally very strong Japanese notion of the Imperial Family being exclusively Japanese.