You're being obtuse again
This really isn't so difficult a point to grasp.
Let's assume for the sake of argument that our ideology is in fact objectively superior. When we then say that imposition of ideology at gunpoint is morally acceptable, we're putting the imprimature of our superior society upon such coercion. Coercion becomes one of the characteristics of a superior ideology and the society it creates.
Thus, "inferior" societies which might be inclined to emulate our superior example will have the perfect justification for their own attempts to impose by force their ideology on others. "Well, America did it, and America's always right! We must be right to do it too!"
Is it sinking in yet?