The Doctor knew the look on Canton's face, it was the one people got when they realised how useful a time machine with an extensive databank could be. It would mean questions. And the Doctor would have to answer them truthfully, because Canton was a good man, and helping them, he deserved a good answer. The Doctor would have tried to give an honest answer even if Canton wasn't trying to save the world, because he believed in honest answers, or at least answers that were in the general area of honesty. Whatever it was, it would be difficult to answer, questions asked by clever people always were.
"So you're from the future."
"Actually, I'm from a planet in the Kasterborus sector, but for the purposes of this conversation, yes, I'm from the future."
"So you know what happens?"
"More or less. I know what happens in the future at this moment in time."
"But close enough?"
"Not really." Canton quirked his eyebrow. He seemed to want an explanation. People always did, despite them never being able to understand them, and the closest analogies were never good enough. "There are some things which are unlikely to change no matter what I do here and now, the sun will die in a few billion years, that sort of thing. Other things," including Canton's own chances of surviving meeting him, but he decided not to point that out, because it was unnecessarily morbid, and he assumed Canton knew that anyway, "for instance who wins the next election, that can still change."
Canton took a moment to assimilate the information. "So if I ask you a question about the future ...?"
"No matter how honestly I answer it, it could still be wrong."
"Okay, I'll be as general as possible." Canton at least seemed to understand the problem. "In the future, does it ever become possible for me to marry David?"
The Doctor wanted to talk about probabilities and matrices, and all of the myriad ways that he could answer, and he knew that wasn't the answer Canton wanted. It wasn't that Canton was stupid; you didn't get to be the man Nixon called when he had a problem if you were stupid. And he knew when fezzes were needed. But explaining things was difficult, people built schemes around the answers to questions like this, built whole lives around it. Casting around for an answer made the Doctor think about casting lots, which made him think of drawing lots, which gave him an idea. "Are you a betting man, Canton?"
"I can be."
"In that case, it's a racing certainty that at some point two men will be able to marry each other in the US. And it's a decent each way bet that if you live another forty years, you personally can get married."
"I've had worse tips."
"So've I. I don't gamble, I always lose my money." Canton caught on to the fact that the Doctor was joking, mostly, and laughed. The Doctor continued, "I could have a look for you, if you wanted, to find out if you do. The TARDIS's data banks, they know everything, or almost everything, to the nearest decimal place."
"No. Thank you."
The Doctor turned to look at Canton. "Are you sure?" It wasn't the kind of offer that people usually turned down.
"Yeah. It'll be bad either way. If you tell me we do get married, I'll spend the rest of my life wondering if I'm about to mess it all up, and if you tell me we don't, I'll spend the rest of our time together worrying about why. It's not worth it."
"You're a remarkably clever man, Canton, did anyone ever tell you that?"