I don't think the "no single mothers" rule should apply to divorced fathers. Reason being that being in that role is a shared experience that can build a connection regardless of whether it is long term or not. The divorced Dad should know all the red flags from his experiences...
If a woman does become long term, you want a woman who has maternal experience. Because this is who you have chosen to be the female role model in your child's life. No single woman wants to mother a frustrated tween as their first foray into parenthood. Just like some one wants a puppy for their first dog, and not some 2year old unsocialized Shelter Pitbull.