Lancelot was not surprised at the 'betrayal'.
It wasn't so much a betrayal as it was really an inability to just let go. He could slip in and out of the picture easily. While he became the centre of attention and conflict when he was there, he would be quickly returned to the back of Arthur and Guinevere's minds when he wasn't there. He did not dominate their lives or even feature much, which he wasn't sure whether he was indifferent or hurt by that fact.
"How could you have said yes to him?"
"I'm frightened, Arthur!"
He had no right to feel hurt—he had played and hurt them several times, and it took him a while to realise that. He knew that the only reason Gwen had agreed to his proposal was because she had had a particularly bad week in regards to Arthur and their ability to be together. Uther was hot on their tails and was probably hoping to belittle Gwen enough so that she would say yes to the first brainless idiot to come along and think she was too 'sensible' to say her marriage vows with two fingers crossed behind her back and one eye on Arthur.
"Frightened of what?"
"I'm frightened that I'll lose you—we keep talking of a future, but will it ever happen?"
"We can't wait for it; we have to make it happen. Both of us."
Betrayal was not in Gwen's nature nor was it in Arthur's. They didn't see it as a betrayal; they were sorry at the possibility that knowledge of their activities would hurt Lancelot, but not sorry they did it. Lancelot knew full well that Arthur had been Gwen's lover. That he still was her lover.
"Do you love him?"
"He's a good man... I cannot hate him."
"But do you love him?"
Lancelot wasn't going to lie. It did frustrate him that upon agreeing to become his fiancée Gwen kept her legs firmly crossed to him when everyone knew she had happily spread them for Arthur when all he could offer her was talk that offered hope. No one knew exactly what Arthur would say but whatever it was clearly made her pliant to his desires. There were all sorts of rumours going around about those two.
"Your kisses always lead to something more."
"I love you."
"Then tell him that."
"I will... I will. He's a good man. I don't want to hurt him."
"He'd be even more hurt if he knew right now you were making love to me."
Lancelot could offer a normal life without any prejudice or strings, a chance to live peacefully with five or six children somewhere, if he could ever get a job or stay still for five minutes. Was that what Gwen was worried about? That he would keep disappearing and coming back?
"We shouldn't be doing this..."
"I know. Kiss me again."
If Gwen would just settle down and marry him then maybe he would settle down too! Wouldn't he? Yes, of course he would. He'd do anything for her. But would he? Yes, yes—and they should settle away from Camelot, away from Arthur. But then would she miss her home and friends? Would she miss Arthur, not just for his love and his body but, for his friendship? Fine, they'll stay in Camelot. But then this would continue...
"We should stop. Wait until he's gone before we do this again."
"We should—ah!—But we won't."
"No, we won't."
Guinevere would never leave Camelot. She would never leave Arthur. He had a hold on her now. She'd keep going back to him. She'd want to go back to him. Gwen knew that just as much as Lancelot did. He could feel it just now when she said she was off to help Gaius with his rounds. Lancelot knew it was a lie—Gaius had told him she didn't need to work today. He knew she was going to see Arthur. But he had pretended he didn't know and let her pluck a kiss on his cheek, similar to one he had seen her give Merlin many times. Did she kiss Arthur like that too? He doubted it.
"Don't stop! Please, don't—ah!"
"I'll never stop."
"I'll—I'll tell him—god! –I'll tell him tonight!"
Lancelot admitted defeat. He hoped Uther would soon too. He wrote an extremely brief note and walked to Arthur's chamber door. He knew they were in there together, making love. If you stood silently by the door you could hear the sounds of desperate whispers, anxious cries and subtle creaks. He shoved the note under the door. He rather crudely thought to himself, that had he not left it somewhere near Arthur's bed Gwen would never find it. Since he could not stick it to Arthur's forehead, under his chamber door was the second best place.
And then he left.
Arthur and Gwen did not notice Lancelot's note for hours. It sat there lonely on the floor as they made love, thinking only of each other. It was only as they were dressing again that Gwen noticed it lying there on the floor. She recognised Lancelot's handwriting and opened it.
"What's that?" Arthur asked over her shoulder.
"I think Lancelot's gone."
Arthur rushed over to look at the note. "He knew we were together?"
"He must have."
He put the note down and wrapped his arms around Gwen. "Are you alright?"
"I'm fine. I'm just guilty he must have known all along."
He kissed her neck. "I love you."
"I love you too."
The note simply read: "You can't break perfection. So I won't."
AN: Written for the A/G community on LJ's 1st drabble tag. The prompt was suggested by a friend of mine as "A/G betray Lancelot."