Sincerely and without a scrap of malice, you've made me completely jealous. This fic absolutely nails something I toyed with when writing my own response to the "infidelity" prompt but which I could not figure out how to convey well and so abandoned. Heh, obscure much? Meaning me, not you.
This thinking that Snape has (as I see it) that what he's doing has no bearing on his relationship with Harry, that he can have his "dalliance" because it means nothing, that he does no harm because Harry doesn't know, seems so spot on and so in keeping, in its way, with what we know of Snape's character, his moral ambiguity, his ethical compromises. I can easily see him thinking (if he thinks about it much at all, which I doubt, because sometimes it's so easy not to) that should Harry find out he (Snape) could fix it because, duh, he loves Harry absolutely without even a smidgen of doubt. Of course Snape being Snape, I think he'd go ballistic if the shoe were on the other foot, but I can see him having some sense of entitlement (not conscious thought, but a sense) to behave this way.
OK, I'm not at all sure I'm being coherent above, but it all comes down to this, for me you absolutely nailed a particular sort of non-malicious blindness to reality and consequences, and I'm metaphorically standing on my chair, applauding wildly, and screaming YES! at the top of my lungs. This is such an excellent take on the prompt and an absolutely awe-inspiring representation of a particular, very real, mindset. And so gut-wrenching in an oddly peaceful way. My hat is off to you. Brilliantly, brilliantly done. Did I mention my jealousy? This is a fic I will read again and again.