Peter knew Severus had joined the Death Eaters, shortly after Sirius' prank (which, incidentally, Peter had done all he could to prevent. He just wished he could have done something before it got that far).
When Voldemort came to Peter, knowing he was the Secret Keeper, and threatened torture, Peter didn't have to think about it too long. His quick capitulation was partly due to fear, partly a building resentment towards the Golden Boys, and partly a hope of impressing Severus.
Unfortunately, Peter hadn't counted on Voldemort wanting secrecy.
Severus never knew the truth. A shame for all concerned, really.
"So, Harry plans to throw the game today? Why?"
"You mean you don't know? I thought it was common knowledge. We've been having a mad passionate affair for months now. Worst-kept secret in the school, and that includes Finch-Fletchley's crush on Snape".
"Is this true, Draco? You're not just having me on?"
"No! He's going to throw the game because I asked him to. He wants to make me happy, or some such, so he's going to let me win for once".
"Well, thank you, Draco. You've just given me the scoop of the year".
"Not a problem, Ms Skeeter".
"What's that? Who's that? Get back here, boy!"
"Oh, shit. My father's going to kill me. He always told me, never get caught".