Ohh, Mystery Author, this is some fantastic, smooth, luscious writing (appropriate for the characters, oh yes), and so bittersweet! The way they come together so easily is lovely. Rosmerta's wonderful with Severus -- knowing when he's in need, accepting him as he is, soothing him while taking as much from the encounters as she can herself, understanding him in a way that only someone who's been paying attention and observing him since he was a kid could.
And the sex...! Guh. Hot. Oh, yes. The contrast between the first time and the last is wonderfully done -- more experienced Severus in the final scenario, both of them older, but Severus still viewing her with the same sort of reverence that he did when he was a teenager, and the mirroring of the positions and such. Really well done.
Also, unf, I might have a little bit of a coming-in-pants kink, so, YEAH.
In reality, it was because he felt like he'd betrayed Lily Evans the moment he'd touched Rosie. And because he was mortified that he'd come in his pants like the school boy he was.