By dwarven tradition, he can never marry. The marriage of a prince, even a fallen one, is in the hands of the Shaperate. Exiled and cut off, there's no hope of obtaining permission now. He tells her as much, as the soft firelight flickers in her hair. They've built a fire a little apart from the others. Her eyes are soft and sad as he speaks. She doesn't try to argue with him, or look for a way around it. She just reaches out a hand and takes his, silently.
He takes a deep breath.
"There's another way," he says. He's thought about this. He asks her if she'll accept his oath as her second, her other half in bone and blood, his honor and his life bound irrevocably to hers. Gorim swore this oath to him once, as he lay in his cradle. They aren't the words he wants to say, but they're the closest he can ever come to those. To do otherwise is to betray his ancestors and surrender his honor, and he will not let her marry an honorless man. She listens carefully, and then nods slowly, her eyes calm and bright.
His hands tremble as he places them in between hers, and the archaic phrases roll down from a thousand generations of his forebears and out upon his tongue. She listens gravely, and when he's done, she takes his hands in hers and kisses them one at a time, gravely. The she places her hands in between his and repeats his words back at him, only stumbling once. He doesn't know if it's even possible for that oath to work both ways, but it's her intent that matters, and his heart sings at the thought that she'd willingly bind herself to him.
That night they sleep apart from the others, making love slowly all through the night, and his sleepy-eyed bleariness the next day is no price at all to pay for the gift she's given him.
A/N: I made that thing about the shaperate up completely. It seemed logical, though, and it also nicely resolved why marriage never comes up as a option for a male character romancing Leliana. I mean, there had to be some reason they weren't falling all over themselves to slap a ring on her...