Why are they telling me what my character will and won't do? Isn't that mine to decide? (It's really the only thing I get to decide.) What happens if my druid changes his mind after seeing how effective breastplate and half-plate are? Do I have to stop being a druid?
As I understand it, to a druid metal is repugnant. It would be like you wearing human skin to work. "Why does society tell me what to do? What happens if I change my mind after seeing how effective human skin is at repelling water?" Just as that phrase (hopefully) boggles you mind, so it is for anyone able to become a druid. If you like metal, make a nature cleric.