Ten/Donna Biodamper
The Doctor made a mistake when he used the Biodamper. He used the wrong one. The one he used is for marriage ceremonies on Gallifrey. Once he put it on Donna and they said what they did, they were married according to Gallifreyan Law, which means she will be his wife no matter what regeneration he is in until she dies. How will he deal with Journey's End now because if he wipes her mind he'll break their bond, killing them both? Which explains why everyone always says that they're married. They are. How do they find out? How do they deal with it? No having them get divorced please.