Section 15. It shall be unlawful for any person, firm, corporation or association, except a dental service corporation, to establish, maintain or operate a non-profit dental service plan; provided, however, that this chapter shall not render unlawful or affect any operation or activity of any company organized under the provisions of chapter one hundred and seventy-five, of any society or fraternal benefit society organized under the provisions of chapter one hundred and seventy-six or subject to any of the provisions thereof, of any non-profit hospital service or medical service corporation organized under the provisions of chapter one hundred and seventy-six A, one hundred and seventy-six B, or one hundred and seventy-six C, or of any corporation organized or to be organized under chapter one hundred and eighty, the existence, purposes, activities and operations of which were lawful or would be lawful notwithstanding the provisions of this chapter.