When I started riding Harley touring bikes, I couldn't stand the foot positioning. And they're much less "foot forward" than the Slim. I'd spent my life riding standard/sport/dirtbike position. I felt unnnatural, and I kept missing shifts.
That said, after a few weeks, the position grew on me. I have highway pegs on my Road glide...and that's waaaay foot forward, and I put my feet up there almost immediately when I go for a ride. I only put my feet on the floorboards to shift...or use my back brake.
Except for: Rough roads, or dirt,, and when I really want to move thorugh the corners. Then I revert back to feet right under me, moving all around the seat and, ahem, trying to get my knee out there (silly, yes, on a Road Glide).
But for most riding, the foot forward position is kinda a gas. It helps immensely to have a back rest. Then you can push against it while riding and give your butt a rest.
Is it natural? I dunno. It's a lot like driving a car. Not like riding a horse. Not at all like making babies with the missus.
One last thing: the heel toe shifter bugged me, until I broke my toe during a trip. I couldn't hardly touch the shifter with out toe pain...but smacking on the heel shifter: Genius!