Do you think that the Japanese would be less inclined to speak frankly about their sex lives with foreign reporters or pollsters than, say, Koreans, Italians or Americans?
Not only about their sex lives, but just about all manner of private things. This is a culture where when you buy a book at a bookstore, they wrap it in a cover for you so that people around you won't know what you are reading.
Many drug stores have extra-opaque dark bags that you can request when you buy stuff you would rather keep private -- don't want the person next to you on the train knowing you have tampons or -- heaven forbid -- hemmhoroid creme. (Of course a small subset of those train riders won't feel any special compunction to hide their 800-page comic book featuring anthropomorphized household appliances coming live at night and raping strangely compliant schoolgirls, but those people are the exception that proves my rule.)
I think the Japanese would be about the least open about the intimate details of their sex lives than basically any modern advanced democratic population.
(The shyness comes from that in a large fraction of quarrels someone(s) will be carried away, in pieces. You don't cause offense without very good reason. Not sober, anyway.)