I really thought that "marriage" was such a manipulative move on the Doctor's part, and unlike when the Ninth Doctor tricked Rose into going into the TARDIS and sending it back to 2005 (he was trying to save her life), here, the Doctor wasn't really thinking about River's feelings or emotions or even her life; he was thinking about the integrity of the timelines. And I think that says something about how the Doctor sees River, and her role in his life.

You know how, when you ask two people what their relationship is to each other, and one says "She's my friend" and the other says, "She's my best friend"? River and the Doctor feels like that.