To his death bed Castiel will secretly attest that Dean didn't mean to propose when he did, the way he did. The older Winchester might not be in to 'chick flick' moments as he calls them but he has a secret romantic streak a mile wide. So Cas was sure that Dean had made plans to 'do it right' but got caught up in the moment.
They were laying in bed, the day long over and the bunker silent as everyone slept. Cas and Dean were spooning naked, Dean pressed up against Cas's back as they both began to drift off to sleep. Dean had his arm around Cas's waist holding him as close to him as possible as he always did. Cas was being lulled by the warmth of Dean against him and the steady rhythm of his heart against his back. Dean was burying his nose in the short hairs on Cas's head until he placed one tender kiss against his neck.
"Marry me." The words were whispered against Cas's skin and even though not the second before Cas was on the verge of sleep he quickly opened his eyes. He wrapped his fingers in Dean's and squeezed.
"Of course." He replied just as quietly. Dean stiffened behind him, which made Cas realize maybe he had thought the other man asleep, and then Cas was rolling over and kissing Dean deeply. When he pulled back Dean was giving him a warm smile and Cas was grinning ear to ear.
So there was no big scene, no prepared speech, no romantic gesture to impress a crowd. Hell there wasn't even a ring, which Sammy would tease Dean about later (Geeze Dean, you proposed without a ring?) but as they lay in the dark, skin on skin and warm under the blankets it was the perfect moment.