You guys! This is awesome--this is what I love most about writing fanfic--talking to folks about things that, you know, I don't have a lot of room to mull over in RL, but only in fandom. :)
My own opinion (and I'm not making any claims just by virtue of having written the fic because I think once it's out there, it's fair game for interpretation by anyone--although I admit to being more opinionated about my own fic, I won't lie!) is that Severus is a bit smug with respect to himself and Harry--but mostly with respect to himself. He knows he loves Harry, without question, without doubt--he would do anything for Harry. Anything. Everything. He does do everything, really--everything he can think of. So he feels...not that it's fair, exactly, but not bad about his thing with the shopkeeper. It's not as if he's ever been spoilt for choice, and it's not as if he's compromising any feeling for Harry, which is what matters, in his eyes. Of course, if Harry found out and was really unable to cope--well, I'd say Snape wouldn't be able to forgive himself. But. Snape has such love for Harry that he hasn't really considered this as a realistic possibility--he's saved Harry so many times he's certain (almost completely!) that he could save this too, if he needed to, by the sheer fact of being Severus Snape and by the sheer force of his love--and he doesn't believe he'll ever need to, because Harry will never know.
Of course, there are moments of doubt, I think. How could there not be? He keeps them spare though, and locked down, because dammit, he's lived a life of apology and wild second guessing, and he won't do it anymore. If the tables were turned and Severus discovered Harry had been doing this--I think Severus mightn't be able to recover. Double standard? Sure. But Severus plays the double standard like a well-worn drum--he's had to live under them his whole life, and this time it's actually justified (in his mind). Harry's so open, see? If Harry did something like this...it would be part of him, you know? Harry's too earnest with too many--not like Severus, who's chosen Harry, only Harry, to willingly protect forever.
At the end, I think Severus finds himself...unstimulated, I think is the right word, by being wanted by someone else. That's what he wants--to be wanted. And yeah, Harry wants him and that's all well and good, but this other man, the other man wants him with a kind of need that Harry doesn't have--and couldn't, because Severus wouldn't deny Harry a single thing. It's kind of circular, I know, but at some point Severus finds that being wanted by someone else like that just...has lost its appeal. There's no real explanation for it--it goes as quietly as it comes, this funny whim of human nature. I guess that's what I feel like Severus acknowledges finally--that he just doesn't care for it anymore; and that he won't, really, again. He just knows. He knows himself; he knows Harry. And he just knows.