searches conducted:
classical greece impiety, classical greece piety, greek religion impiety, greek religion propitiation, greek religion hilasterion, others I can't remember right now.
I've gotten a couple of good resources on piety in general, e.g. page 23 of this, but none on this specific offense.
I also looked through Greek poetry (esp. love poetry) online to see if there are any examples of lovers doing this--if there were, then it's probably not really an offense--but the closest I can find is Sappho's statement "like unto the gods he seems to me". (which Catullus apparently found borderline impious, but he's centuries later and Roman)
question: Would it be impiety for a person to refer to another person as one of the gods (meaning it metaphorically, but saying it literally)? If so, how bad of an offense to that god would it be, on a scale of "quite minor" to "live in fear of lightning bolts"?
(scenario: A & B are lovers, and are quoting some bits of poetry about Aphrodite to each other. And then B actually calls A 'Aphrodite.' How worried/freaked-out might A be about this, if they're a fairly pious sort of person?)
