All forces acting on a body at any instant can be resolved into a single force vector.
If the aircraft is at constant altitude then the weight and lift forces cancel out.
If the aircraft is flying 'straight and level' then the lateral forces must also cancel.
And if it is travelling at constant speed -- and thus not accelerating -- then the longitudinal forces cancel.
Therefore there is no overall force acting on the aircraft. Perfectly in line with Newton's first and second laws.
QED
