Sam was the first to notice the slight change in Deans behavior whenever Cas was around. He kept quiet about it but after the Mark of Cain was no problem anymore and the darkness was tucked away once again he couldn't do that anymore. So he asked Dean what he truly felt for Cas.
Time had passed and Dean and Castiel were a couple now. They were still hunting but it felt good for Dean to know there was someone waiting for him in the bunker. They were still in love like in the beginning. Sure they fought sometimes, but who doesn't? In the end they are always there to protect each other, and Sam knows. It never felt to him like his brother abandoned him to be with Cas. Cas was like a brother to him. He loved them both, and they truly loved each other.
Sam sat in his room when he heard loud footsteps outside. He stood up and opened the door to find Cas running down the hallway, literally running. The angel stopped at his and Dean's bedroom door and knocked like a mad man. Sam silently watched as Dean opened the door and Cas literally jumped into his arms and kissed him.
At first Dean seemed to kiss back but then he gently shoved his boyfriend away. „Wow Cas, what's up? You okay?" Sam smiled at the concern in his brother's voice.
Sam saw the smile slowly growing on Cas' face. „Love wins, Dean."
„What 're you talking about?" Dean obviously relaxed, softly smiling down at his angel. „What does that mean?"
„I love you Dean, you know that?" Cas continued as Dean nodded and Sam wondered what was going on. „I flew to D.C. as the Supreme Court legalized homosexual marriages five minutes ago. What I mean is" he stopped and pulled a small black box out of his trenchcoat pocket, „Dean, will you marry me?"
Sam's eyes widened. Did Cas just seriously said what he thought he did?
Dean seemed shocked. Sam understood. As a hunter, your chance at having a working marriage even as a heterosexual isn't very high. Hell, your chance at surviving until you're thirty isn't very high, Sam had his experiences with that.
„I …", Dean stuttered. „Cas that is …"
„It is totally legal", Cas added. „In all fifty states."
Dean laughed nervously. „That's not it. I just, … I ..."
Cas' smiled disappeared. „You don't want me", he stated to himself.
„No!", Dean interrupted. „Hell, Cas, you're everything I ever wanted." Sam watched as Dean pulled Cas into a deep kiss. They stumbled back into the bedroom and the door closed.
Seemed like Sam got a wedding to arrange.