Note: I do not own Supernatural of any of its characters. Some overlap with the books.
It doesn't start as something romantic. In fact, it starts as probably the second most awkward, embarrassing, nonsexy situation ever. It's five weeks after Gabriel managed to pry the gate to Purgatory open, tugging a barely alive Castiel with him, four weeks after Dean realized that what he had been feeling for Cas was actually love, and three weeks after Sam and Gabriel had decided, what the hell?, and started sleeping with each other. They should have all lived happily ever-after. But when were their lives ever that simple?
The killings in Salem had started up again, so they had headed back to make sure that the ghosts hadn't missed any witches. Of course, there were still a few left, and they were intent on getting revenge for the deaths of their sisters. With Gabriel and Castiel's help, they had quickly located and ganked the remainder of the Coven, but not before the leader had cast a spell on Dean that meant that if he didn't have sex with an archangel within the next 24-hours, he'd die.
It was probably meant to be fatal; it wasn't widely publicized that Gabriel had been resurrected. But thankfully, they had a way to break the spell. The problem was that Dean had refused to do so until, quite literally, the last ten minutes. It didn't matter that both Castiel and Sam had said that it was okay and Gabriel was willing if Dean was; Dean had insisted that he'd be betraying both his lover and his brother. There had to be some other way. But as the time dragged on and none of them could find a way out of it, Dean had started to get weaker and weaker. Fifty minutes after the spell had been cast, the elder Winchester lay on the bed, convulsing and foaming at the mouth. The other three had gathered around Dean, begging him to let Gabriel have sex with him. Whether it was a nod or just a convulsion, they couldn't entirely be sure, but it was good enough for them.
So now, they all lay on the bed. Castiel lay on one side of Dean, kissing his forehead and telling him that it was all going to be okay. Sam lay on the other side, holding his brother's hand and letting him know that they'd get through this; that none of them minded. Gabriel avoids everyone's eyes, snapping away both his and Dean's pants. He doesn't bother with foreplay; he doesn't have time. He uses his angel mojo to prepare his lover's brother and then slides right in. The archangel's acutely aware of Sam and Cas looking anywhere but him and of Dean closing his eyes. He finishes in as few thrusts as possible, and withdraws as soon Dean starts to return to normal. Gabriel snaps them clean, replaces their pants, and gets up, feeling that it would be best if he leaves. He's stopped by Sam.
Sam knows that they'd all rather leave and forget that any of this ever happened, but also knows that they can't. Dean's busy chanting he's sorry, while Sam and Cas both assure him that he's done nothing wrong. Gabriel ends up curling up against Sam because, while he'd like to comfort Dean, he doubts the older Winchester's ready to face the archangel, and while he'd also like to apologize to his brother, he doesn't know how or what to apologize for. So they all curl around each other, one big ball of vulnerable souls and graces, knowing that, if even one of them leaves, they'll all fall apart.