So Severus had no experience. The boys talked about this a lot actually but they also talked about a lot of thugs that made them seem full of shit. Severus had trust issues. He didn't trust Sirius, not for a moment. But he also had pride issues and he didn't want to back down from a challenge.

It was against who he secretly was, the Dark and powerful Half-Blood Prince.

"You're so damned worried about my virginity. I'll get rid of it then."