Scholars have been offering re-interpretations of the "turn the other cheek" lesson from the Sermon on the Mount for decades.
The reason they thing that it must not be literal is because no nations, primarily Christian, or otherwise have been able to follow that advice.
One opinion is that in Jesus' time, the left hand was not used to strike (nor was it used for other useful things), and by offering the other cheek (after a presumable right handed slap across the left cheek), the attacker would not be able to strike again.
I think that theory is horsecrap.