He forgets sometimes, what with all of her Time Lord-y cleverness and all of that hair, but she is so, so human.
She is River Song, complicated time-space event, traveler of the universe, Child of the TARDIS, but she is also Melody Pond, daughter of Amy and Rory, the most humany humans he's ever known.
She stands in front of him, defiant and beautiful and wrong, and he sees the humanity coursing through her veins, pumping in even rhythm through both of her hearts. She does not flinch. She will not back down. She is River Song and she will love him until the very end of time. She will, she has, she does.
It is not right.
It is not okay.
Billions upon billions. The entire universe. No one person is worth that, least of all him, he who has caused the suffering and death of so many already. Another world will collapse and he won't be able to turn away, won't be able to avert his eyes as he did when Gallifrey died. It will swirl around him and over him and through him, the apocalyptic epicenter, the end of a world that is wrong and a time that doesn't exist.
All because of her and because of him and because of a them that he is only beginning to understand, a them powerful enough and precious enough to root her in this foolishness, to allow human emotion to overcome her Time Lord brain.
She claims that she will suffer more than the entire universe if she kills him and he thinks she's never been more selfish or more honest in the entire time he's known her. It's frustrating and difficult and mad and he doesn't know what to do about it because she's crying and he hates it when she cries but she is still so selfish and so wrong.
He turns away.
In this moment, this never-ending moment, she is humanity itself, the very best and the very worst. What is humanity, if not the culmination of a selfishness to end all time and the biggest love in the universe?
He almost laughs at the absurdity of it all; the two of them, standing on top of an Egyptian pyramid with her parents, in the middle of the end of the universe caused by a spacesuit and a robot and a plan gone awry.
When he turns back around, decision made, he thinks that it was stupid of him to try to do this without her anyway, to ask her to play her role without disclosing the plot, but he's still so new at this, this not being alone thing, and he forgets sometimes that she's there and that really shouldn't leave her out of things. And then she does things like this, like blowing a hole in the universe, to remind him that he couldn't even if he tried.
(She will tease him later – "Of course it was all your fault. You were the one who thought it would be smart to keep secrets."
He will retaliate – "What happened to all of that human 'love, honor, and obey' rubbish? This was the part where you were supposed to obey."
She will smile – "Those weren't the vows I took, sweetie.")
He really ought to hate her for it.
Mad, impossible River Song with her cleverness and her hair and her human failings, refusing to take no for an answer, refusing to let him face his death alone without knowing the love of the universe or of her. He ought to hate her.
But instead (because she is so, so human and so selfish and so wrong and so very much his equal), he pulls the bowtie from around his neck and wraps one end around his hand, extending to her a piece of cloth and a hand and a promise that he hopes he'll be able to keep.