Or men tend to pair off M/M more often than women do, or this culture has very little pairing at all, or... So many possible reasons.

It makes me curious whether anyone has done an index of spec fic stories based on what the dominant family structures are.

ETA: Hah! I couldn't find it when I was commenting before, but this is what I was thinking of: http://www.uchronia.net/bib.cgi/diverge.html, the store that lists AU stories chronologically, by when their universe docs from ours. I'd been wondering if anyone has done something similar with stories, grouping them by the most-common family configurations of each ficton. Probably not: it would be reductionist. But interesting.