Given the dearth of women among the Inklings, the scarcity of female academics during Tolkien's time, the proscriptions against female travel (the Grand Tour of Europe was gentlemen's perogative in the eighteenth century but not a ladies') and the absence of married couples in most epic adventure stories (although not in folk tales), is it any wonder there are no married couples? Domestic concerns were never really part of high literature unless it was the likes of the (adulterous) medieval romance tradition.
After all, would it be conceivable to interrupt the quest for childbirth?
This is actually one reason why I enjoyed the Patrick Rothfuss book,
The Name of the Wind: it has such well developed early episodes about a travelling troupe with families et al. Sure, they aren't on a quest adventure thingy, but the book does present a full social context before the hero is orphaned, like so many Victorian heroes and heroines (sheroes?) are. That Victorian literary tradition might also have an influence.