The speaker tries to explain a relationship by comparing it to punctuation, and the comparison turns out to be surprisingly flexible. Commas stand for pauses, periods for stops, semicolons for the ways the two people stay linked. Exclamation marks suggest sudden moments that could end the sentence altogether. The question mark is the most honest one: the speaker admits things feel uncertain, yet still hopes "we can probably make it through this."
The phrase "black and blue" hints that the relationship has left some bruises, so the tone is a mix of tenderness and hurt. Even so, the speaker sounds hopeful rather than defeated. Growing apart is treated gently, like different sentences that mean different things but still belong to the same piece of writing, joined like strings. Repeating the opening couplet at the end gives the poem a sense of acceptance. Whatever happens, the speaker sees the bond as lasting, imperfect and real.