I love how the settings for the story are so ordinary and how the interactions are similarly prosaic. It makes the sharpness of the infidelity -- and of your prose -- stand out all the more.
The fangirl in me cries out to quote this back at ya: Severus felt himself widen, further than before, his heart thinning at the edge into an inexplicable membrane of space and inconsequence, momentary but for the warmth of the hand now fitting to his. I'd quote the whole last part if I didn't fear redundance and character limits on comments, because I've never seen infidelity resolved in such a quiet, isolated way.