Sirius simply eased that finger into Snape, letting it rest there as he kissed a few more kisses to Snape's spine. Sirius was read up enough to know that if he did'nt prep the other boy, at least a little. He could hurt him. And not the good sort of hurt. He slowly began to ease that finger out till it reached the tip before he was easing it back in .

He gently kissed and bite at his back to distract him from the pain. To sooth that discomfort. As eager as he wanted to simply mount the other boy and ride him till he came. He ad to do this like this. Snape had to trust him enough to know what he was doing.

"Hush Severus"