River looked at her, stepping closer. Looked straight into her eyes. And took a breath to speak.
Then, he blinked. Something in his expression softened, withered. "Oh." A slow smile appeared on his lips. "Captain Rogers. By the authority vested in me, blah blah Holy Roman Emperor, blah blah blah, I authorise you to marry us."
Steve blinked. "What?"
River looked back at him, frowning. "You want to save the universe? Then marry us. Quickly."
Steve looked startled, but said, "I now pronounce you husband and wife."
River Song extended his hands to his new wife. "Allow me to kiss my bride."