The poem is a tiny thank-you note, and its surprise is in what it thanks. It starts out sounding solemn, with "love and lasting kindness," the sort of phrase you might hear in a toast or a prayer. Then the tone drops into something earthier: "how much we drink and stink / mouth to mouth." The speaker isn't polishing the picture of a good marriage. He's happy with the messy, bodily, slightly smelly side of it.
The title sets up a joke. A "threesome" sounds racy, but the third party turns out to be a bottle shared between husband and wife. That twist makes the poem funny, yet the humor doesn't undercut the feeling. The speaker seems content, comfortable, and a little cheeky. Judging by the language, long companionship has made room for ease and honesty, even for stink. The mood is warm, playful, and quietly grateful, a small celebration of ordinary closeness.