Sirius gently stilled Snape's hand at those words, his eyes full of surprise. Did Snape seriously just suggest what he thought he did? Sirius could make Severus trust me, a least a little. If they kept doing this. He'd have to at one point. Trust him not to leak the secret.

"And how, do you propose you do that?" he said, titling his head just a tad to expose that mark on his neck. Almost as it help Snape make a choice.