They didn't plan it at all--not in a big "florist and photographer and minister" way, but not in a little "intending to say something before you actually say it" way either. They never did, not even the not-technically-the-first time.
It happened one evening, when the now was quiet and still and the little blue book was beginning to get battered. He had been her pick up--or, more accurately, getaway--from a prestigious Bilvocoparian university, where she had first given a seminar and subsequently broken into a confidential research archive. Now, after a too-close escape, the loss of her suitcase, and a round scolding from the Doctor, they sat in the doorway of the TARDIS with their legs hanging out into space and watched a nebula slowly boil.
Although the changing colors looked as though they ought to produce music, the vacuum of space was perfectly silent. All that could be heard was their breathing and the faint hum of the TARDIS. Then, without preamble, the Doctor began to speak, in a low, melodious voice as though he were reciting alien poetry to the alien vista. His tongue curled around the words and rolled them about in his mouth and bounced them off his teeth.
"What does that mean?" River asked him when he had finished, because he wanted her to ask.
"Well, allowing for cultural contextual equivalents and some grammatical compression," he said quietly, "It's Gallifreyan for 'Do you take this Time Lord as your lawfully wedded husband, for better or for worse, for sooner or for later, in regeneration and in health, as long as you both shall live?'"
"Yeah," she answered, as solemn as he. "And do you take this woman for ginger or for blonde, for happy or for faking it, and however else she might pop up, as long as you both shall live?"
"Yes," he said.
She took his hand, stood, and led him away. They honeymooned in the TARDIS conservatory.
Later they lay on their backs and looked up through the branches of a toffee-fruit tree from Ellipsis 9, at the artificial sunlight shining through the glass-paned ceiling.
"So am I Mrs. Smith, now?" she said.
"No," he replied immediately. "Smith is a rubbish name." He pillowed the back of his head with a hand. "I like Song. You should keep Song."
"What, are you going to be Song too, then? 'Cos I love you, sweetie, but I don't think you could pull it off."
He looked over at her and grinned. "Yeah, me neither. Mister Song sounds like I ought to host a children's television programme. And Doctor Song is... well, you."
"Better believe it," she said, poking him in the ribs as she turned over on her side and laid a head on his shoulder. "So I've got my maiden name and you're nameless. Quite the couple we are. Whatever shall we put on the mailbox?"
"Well," he said, "not nameless." And he lowered his lips to her ear and spoke again as he had before. The syllables twined and looped around each other and into her ear. There was less than before--less like the recitation of poetry and more like, she thought, the title of a piece of music.
"What's that?" she said.
"It's my name."
"Bit of a mouthful. No wonder you don't use it."
He was quiet for a moment.
"The Time Lords took it from me when they exiled me," he said. "That's why I don't use it. A Gallifreyan name is made up of all your status and heritage and who you are... so the nearest thing they could do to stripping me of my identity was stripping me of my name."
"But they're gone now."
"Exactly," he chided lightly. "Nobody left to give it back." He breathed in through his nose. "My family were already gone when I was exiled," he continued. "All except Susan." River nodded against his shoulder. She knew about Susan. "So she, as the last member of my house and family, was the only one who could 'keep care of' and use my name, by law. The Time Lords always were big fans of solidarity, you know. They loved families. All that 'stick by your own,' and then the association with status and blood and--"
"I'll keep good care of it," she said.
Another silence. Then he nodded too, and wrapped his arm around her.
It took a while, but she finally convinced him to drop her off at her flat (which happened to be outside of Bilvocoparius jurisdiction). He sulked, but let her leave when she agreed to go with him to meet Gary Gygax as soon as she'd finished publishing her research.
"I wonder if they'll return my suitcase?" she wondered aloud. "Losing it is such a bother. I had quite a few valuable papers and things in there."
"I seriously doubt it, what with the multiple felonies you committed before leaving!"
"Oh, don't start." She kissed him and gathered up her ill-gotten papers. "Oh well. I didn't have a luggage tag on it anyway." She winked and walked out, waving over her shoulder.
It wasn't until he'd taken off again that he remembered something he'd been meaning to ask before the kiss. He popped the TARDIS right back in front of her door and stuck his head out just as she was fishing in her satin purse for the key. She looked up and seemed surprised, but smiled.
"The thing is," he said, "what if one of us meets a version of the other that hasn't done this yet? Are they married, too? Or just the one of them?"
She smirked and stroked her chin in thought. "Well," she replied, "I suppose that would depend upon whether the younger one said 'yes,'" she mused. The Doctor pricked up and pointed a finger warningly at her.
"Hey! Hey, now, you can't... we can't go around proposing to each other's earlier selves! You should know better than--"
"I'm not saying anyone needs to propose anything," she interrupted smoothly. "All I mentioned was a 'yes.' I certainly don't care how you happen to get it out of me."
While he was working that out, River retrieved her key. She stuck it in the door lock and turned it, then tested the knob and put the key back in her purse. Something about this seemed off to the Doctor, and he squinted at her in confusion.
"Wait. Why do you have your suitcase? I thought you lost it at the... Why do you have your suitcase?"
"I've got a archaeology seminar at Bilvocoparius tomorrow. Nothing big, just a little overnight trip out-of-system."
"Archaeology seminar. But..." A pause. "Right." He looked from the suitcase to her. She was grinning. He sighed. "Bugger."
"See you soon, sweetie."
"Right. Put a luggage tag on that thing, would you?"
"Doctor?" she said, just as he was pulling his head back into the TARDIS. He stuck it out again, a little irked.
She came over, soon-to-be-lost suitcase and all, and kissed him on the cheek.
"Yes," she said.
A grin popped up on his face in spite of himself.
He popped back in and disappeared. River Song pulled her key back out to go back inside and get a luggage tag.