I have some new information on this topic. For weeks of game play I only ever experienced the automatic townie generation creating two types of "Not currently in world" households. It created single sims households by and large and added in families structured as single mother/teen daughter/child son once my own sim family started having children. I saw this problem reported by many others on the forums so knew that it was not only happening in my game. But I also saw people say they were getting a more random set of families that were more balanced that included two parents and some female children which I never saw in my game. From this I took away that there was a difference in game play that was triggering the types of townie families being generated.
Today for the first time I had a game in which the game created something other than the Mother/teen daughter/child son family structure. My game still generated those families but for the first time I saw it create families with two parents and some female children. I started a new game and for the first time I decided to play the Goth family. I started up the game, dropped in a sim into a starter home from CAS and immediately switched to the Goth household. Saved the game while it was still 8:00 am on the sim clock, went back to manage worlds and made sure that the Goths were my only household in the played list by marking the sim I started the game with as unplayed.
I then started up live mode with the Goths. Because Bella has the social aspiration I had her out on the street greeting anyone who walked by. So I noticed all the townies walking by. To my surprise I saw several female children walk by who were not Olivia Spencer-Kim-Lewis. The first time ever I had seen in my game female children townies that I had not personally added myself.
The only difference that I can think between this game and all my others is the start. In all my previous games I had started out with a single or couple of adult sims and it wasn't untill a couple of sim weeks had passed before they began to have children. I do know that when your own played sim households have children as soon as those children age up to child the game starts adding in townie families for that new child to have other children to play with. But in this game I was starting out the game with a family that already had a child from day one - Alexander Goth.
When a new game is started fresh the only households in the game are the one you add from CAS and the default inhabited households. There are no townies generated until live mode is started. My conclusion is that the types of townies created are triggered by the family you start play with.
I don't know why when I play a game that starts with a single or more adult sims only the game gets stuck on that single family stucture that never produces female children but it does. I would prefer it if the game never created that single mother family structure but created the old fashioned adult male/adult female with a couple of children. My reasons have nothing to do with stereotyping but I feel that that structure of family will bring a better balance in the genders in all age groups and will not create towns over populated by women but never any female children.