It's interesting to me how many people have said that they don't like or don't read infidelity fics, and I noticed in one of your comments that you said it's a bit of a fandom taboo. Which interests me because, though I'd never really thought about it, I think you're right. (And also about Ginny, because that is a favourite way to break up H/G, isn't it?)
I actually like infidelity fics. Or I like the idea of them; I can't think of any that I've really liked. (There was a Harry/Ron one, once, where they were married but had their thing on the side. I remember enjoying that, but forget anything else about it.) What I like about them, usually, is the forbidenness of them, which adds a certain spark to the situation. (Unless it's infidelity with a focus on angst.) So this story was quite different from most of what I've read because it wasn't about the angst or about the extra spark. It was just sort of... this is one bit of Harry and Severus' life in a way that feels very true to life. I think dozens of affairs like this must begin and end without anyone ever finding out. I like that your resisted the chance to let Harry find out - it put a different spin on the story than it might have had otherwise.