Check this out:
do you sleep in the same bed - Google Search
There's a lot of people answering this very question on a lot of those ~answer websites. I'm sure there's a good number who do sleep separated, but I'd bet that the majority of couples sleep together. I realize the google results there might not be the best quality results you can get, but I think they show a somewhat decent example-- and keep in mind to find those questions to answer the searcher would first have to be concerned about that situation (at least i'd suspect most of them would be)... so I'd say there'd be an advantage on the people who DON'T sleep together on those answer sites.
At any rate, do you think that Japanese don't sleep in the same beds as often as "westerners"? Do you think the survey could be flawed and that a lot of these cases are people who just have two futons purshed together? (think of like two twin beds pushed together to make one bigger bed, that's kind of how I picture it but with futons).
I think if more Japanese do sleep separately then it might be related to the work culture more so than lack of affection.