I believe the SF'ers are "naturals" in the sense that they have a high aptitude for martial arts. Ken trains alot, he may have a wife, but he trains, and he has a student canonically. He's always trying to keep with Ryu, and he trains to reach that goal. Ryu trains to be the best, that's the main difference between the two. Like the turtles, they undergo serious training, but Ryu takes it to the next step. If Ryu and Ken were to have to learn a technique, Ryu would learn it first regardless, it's simply in his body. Which is why he trains more to master it.
It's not like Ken is a 2 and Ryu is a 10 in training IMO. More like Ken is an 8 compared to Ryu's 10, or Akuma's 10.
It would be cool to watch Bison train though. Also (canonically) Bison wanted Ken's body. Oro wanted to train him, and Akuma saw his potential.