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Sam couldn't believe his eyes. There was Dean..on top of Rhonda Hurley. Dean had promised that if they were going to break all moral boundaries it would be together. It would be just the two of them. Dean swore all the flings would stop. Yet, here he was with his mouth on her neck and his hands roaming her body.
Sam let out a tiny gasp as Dean moaned. That was supposed to be Sam under him. They had made a pact. He swore.
Somehow, Sam couldn't help but smile though, because Dean was still gorgeous in the way his body rolled and the way the sweat along his back shimmered. Dean's hair was a matted mess and his lips were a cherry red. He was beyond beautiful and Sam loved him so much it hurt.
Cas was watching the scene play out. Just as he had watched both boys' entire lives. He new this was a big moment between the two. This would be the reason for Sam's leaving. And it would begin the downward spiral of Dean Winchester. Until one black haired blue eyed angel would save him.
This was Castiel's plan all along. Using Cupid to create a desire for Dean that Sam could never satisfy, and breaking the strongest bond the angel had ever seen.