There was only one reason for a married couple in the TARDIS. In the UK, this is a children's show. In a children's show, in order to remove any possibility of scandal, it's necessary to marry the characters. Rory and Amy were married to produce River.
Similiarly, there is only one reason to marry the Doctor.
I think Clara's identity follows logically if you wonder why, after almost fifty years, they would marry the Doctor.
Good grief. You, along with the person who suggested it was Susan, are completely forgetting that they kissed
, that Clara has been extremely flirtatious with the Doctor from the get-go. By your own logic, the last
thing they would do in a children's show is include incestuous themes! Of course she isn't his daughter or granddaughter or any kind of relative! Ick! Eww!
Fans have been drawing weird connections like that for years. For instance, after the McGann film, a New Zealand fanzine argued that Grace was the Doctor's half-human mother