I didn't say that children would be better off every time with their dad, just that, in general--i.e., most--are. One reason I've heard as to why is that dads are generally better off financially. I would assume that a big reason is that the man is often more psychologically stable.
Maybe the reason that men are allegedly more psychologically stable is because they are paid more. Financial instability is very stressful. Plus, more men than women are pedophiles and serial killers. Don't forget that. :-)
Me, I think kids need to grow up with a mom AND a dad. Married, stable couples are the ideal...and if the marriage doesn't work out, joint custody should be the default. That's just my opinion, though.