Factoid: Single mothers are extremely numerous in La Paz Mexico. Single mothers try to maintain their "girlish figure" for a long time; I know several mothers pushing 45 who have not blown up to be fatties. Conversely, or "on the other hand", married females about that age are generally fuckin FAT. They can all keep their figures if they make an effort; check out Jane Seymour at age 67 was in Playboy, and she's 70+ today and still looking good. Anyway, the downside of single mothers is obvious; you will not be a priority to them, which is OK, but also they're frequently busy doing something related to the kid's school, sports, friends, father, grandparents, etc.
* Crime
* Poor Educational Outcomes
* Poverty
* Child Abuse
* Dependence
* Less Likely to climb Economic Ladder
...so how come society isn't aggressively disincentivizing it?
And no, it's not poverty. Even well-off single mothers underperform two-parent families with less household income.
http://www.slate.com/articles/double_x/doublex/2012/07/single_motherhood_worse_for_children_.html