1. No police man can take unto their hands more so when no crime was committed.
2. there is no law prohibiting a couple in wandering the streets if the same is not causing any nuisance or disturbance to the public.
3. Since none was committed by you they committed crime for which they can brought to books.
4. So lodge a complaint to the police station where this incident happened. If you did not know their names then also there is no problem as they can find out their identity form the duty roaster.
5. After lodging complaint with the local police bring this to the notice of higher officials also.