The poem is about the quick, polite kiss people exchange as a greeting, the kind you give a friend or acquaintance without thinking much about it. The speaker spends most of the poem saying what this kiss is not. It isn't "an open door," so it doesn't invite anything further. It isn't a "hope preserve," a nice phrase that suggests hope kept in a jar for later. And it isn't "sin to us," so there's nothing guilty or forbidden in it either.
What's left is the last line: the kiss is "fleet / home and then off." It lands for an instant and is gone, like a bird touching down on a branch.
The speaker sounds calm, almost reassuring, but the long list of denials makes me wonder. Why spell it all out unless someone might be tempted to read more into it? The mood is light and brisk on the surface, with a faint wistfulness underneath, a sense of how much a small gesture can hold back.