Answer

We are certainly not commanded to forgive in such a situation. I also think it is not appropriate to forgive. Why forgive if he did not ask for forgiveness?!
However, one can distinguish between cognitive forgiveness and emotional forgiveness. There is value in someone who does not get angry and does not let things affect his feelings. But in principle, there is no reason to pardon someone who did not ask forgiveness. Perhaps this is what the Sages meant when they said: any Torah scholar who does not take revenge and bear a grudge like a serpent is not a Torah scholar. (Though according to this, it is not speaking only about a Torah scholar. In the conventional interpretation, it applies only to a Torah scholar because of the honor of the Torah. But notice that this speaks about revenge and bearing a grudge, not about inward forgiveness. See below.)
The question of turning the other cheek is a matter of behavior, not of feelings. A person can stand up for himself and still forgive inwardly, and one can also refrain from standing up for oneself and still not forgive inwardly. Revenge and bearing a grudge are also matters of behavior. Not taking revenge and not bearing a grudge means not giving expression to the anger in your actions, but it does not necessarily mean not being angry in your heart.