From a mechanical engineering point of view, it is possible to design the wing to be stiff so it wont flex. But it would be heavier and more bulky. If you can get away with some flex, it would reduce the weight of the aircraft, which is important for a combat aircraft. Also modern wings, especially when made with composites are designed to flex in flight for better aerodynamic performance. Just google Boeing 787 wing flex. It is meant to mimic how bird wings looks.
A wing that won’t flex require material with infinite modulus of elasticity, Which, not surprisingly, do not exist.