I think what was going through Snape's mind when Harry told him was *headdesk* *headdesk* *headdesk* and "damn it!" If I remember correctly, and it's been some time now since I read the fic, Snape never says "it wasn't me" or anything like that. Maybe you're thinking about the immediate aftermath of the first time they have sex in a horizontal position and Snape apologizes and says something on the order of, "I'm no good at this. This isn't what I do,"? If that's the part you're talking about then in my head Snape means he pretty much only does the anonymous bathroom sex, he has no real experience, or at least no recent experience, of "making love". The only exception would be Walter, but Walter is a Muggle and basically just a toy, not an equal, no real affection and no real attempt at give and take. I could be wrong. This is Snape we're talking about, and who knows what really goes on in his head.