"My grandparents are not married."
Translation:Besteforeldrene mine er ikke gift.
One couple is one marriage, making it singular. You will very rarely come across the plural form. You can divide groups into married and unmarried couples, Gifte og ugifte par. If you had some friends over and they were married but not to each other, it is still singular. De er gift men ikke med hverandre (They are married but not with each other )