OK, this is happening in my game. I just thought it was crazy that I had not seen 1 girl child generated (unless it was in a played household). But I am getting the mother, teen daughter, boy child theme in every unplayed household (other than tons of single Sims) I see in the world list. I'm also seeing the diversity within these families that has been remarked about.
However, I'm not sure the gameplaying approach is making the difference here. In the old world I played a bit legacy-style. Then I started a new world, playing the Friends characters, and hopping around between 4 households for a few Sim weeks before allowing them to start families. And this not-so-random family theme has been occurring in both of these neighborhoods.
Here's a screenshot from my game. Coincidence?
