So Lesbians in Japan do what Gay Men used to do in the U.S. in the 50's? With one expected to be effimine and keep house, while the other was supposed to be manly and act like the stereotypical hetero male of the time? Only, of course, it's a reversal of that 50's senario; in the gay male senario, only one was allowed to act "manly" while in this case, only one is allowed to act "feminine."
Reading about this right now, lots of articles on eastern culture and Japanese societal structures and ideology (in an etnographic sense of the word). Not about gender roles per se, but I can see how it applies...
Norms and structure seems to be a corner stone in a Japanese's conscoiusness in a much more profound way than in the west. They're brought up on routines, norms, and a strict social code on how to behave and how certain social patterns look and feel.
So when something major chances, like entering a relationship, I guess that for a lesbian (or gayforthat matter) couple, it's easier to adapt the situation to the familiar patterns, than to find a new pattern for the new situation.
From what I understand in Japanese culture, (this goes for Chinese to as far as I know) a man or woman is not considered a 'success' unless they are married. Their standing in their family and even advancement at work can all have bearing on if they are not married.
From what I understand, there are actually many cases of Gay men getting married to Lesbians and even living together with the understanding that it's only for appearances. Both partners go out with otheres but live together to keep up appearance. It's a semi accepted method of dealing with the issue without really dealing with it.
The key concepts to understanding this facet of Japanese culture are "tatemae" and "honne."
"Tatemae" is the reality that you present to the external world. It's more than just an image you project. Tatemae is ACCEPTED as being "truth" even though everyone knows that it's just done for appearances. "Honne" is your actual, internal truth...the one where you are true to yourself. It is the reality that you experience internally.
I truly believe that from a cultural point of view, both concepts represent "reality" in traditional Japan. It makes no sense to most Westerners that you can have two parallel realities in a society. But, as Karel van Wolferen pointed out, the concepts of logic from Aristotle and Socrates have existed in Western societies for thousands of years...but were not introduced into Japan until the 19th century.
I've experienced this too many times to think that it's otherwise. "Reality" in Japan is malleable. So the situation you describe is similar to ones where the husband has a mistress but no one talks about it...or where the wife goes out while the husband is at work to make money as a prostitute. It's more than the traditional small-town attitude of looking the other way...these are behaviors that are completely accepted, and therefore completely ignored. Similar to the Japanese saying, "Nudity is often seen but seldom noticed."
Also, keep in mind that Japan is a matriarchal culture, and women have much more power across society in general than men do.