Perhaps I'm too far out of my element here, but what makes you assume that Western couples didn't usually sleep in the same bed? The vast majority of families would not have been able to afford individual beds for individual family members until very recently. I'm actually pretty certain that not only did couples share beds out of necessity in pre-industrial Europe, that in many (most?) cases, children shared beds too, not uncommonly with the parents as well. During earlier times, it would not be uncommon for livestock to sleep in the same room as well. And for the most part, this was a result of being too poor to be able to afford better accommodations.

I can think of two reasons why one might assume that western married couples slept in separate beds: 1) That's the way that TV couples are portrayed until the 80s and 90s or 2) Because royalty (and perhaps rich aristocracy) kept separate apartments. I could argue that neither of those groups are anything close to representative of the general population, however.

Source: At Day's Close: Night in Times Past by Roger Ekrich. Here is a Sunday Book Review of the book from the New York Times.

The most recent Western vogue for twin beds among middle class couples ran from the late 19th to mid 20th century. Couples adopted this new "modern" fashion for two reasons. First it was considered healthier; there were many 19th century hygienic writers who pushed for everyone having their own bed for better sleep and less disease transmission. Secondly by the 20s and 30s there was a political aspect: men and women were more equal. By having (identical, equal) "modern" "twin" beds couples showed that their marriage was based on more than sex--companionship and shared interests in an equal relationship of autonomous partners.

Some modern historians argue that we shouldn't lose track of the positive way 20s and 30s people saw twin beds, in contrast to the modern idea of "Twin beds mean a troubled marriage!" or "No one really had or wanted them, it was just for movies and television." See Hinds, "Together and Apart: Twin Beds, Domestic Hygiene and Modern Marriage, 1890–1945", Journal of Design History.