What are the binaries (oppositions like short/long) that operate in "Sonnet 18"?
I think the binaries operate more like comparisons, and at times, oppositions. The speaker compares his muse or the object of his affection with summer/nature. But he compares the muse as she is in the poem - because the poem will live forever. This gives the object of his affection an immortal quality, recalling Plato's perfect Forms and the ideal of Heaven or Earth before The Fall.
The muse will live forever in the poem, but the summer's "lease hath all too short a date." And summer and nature will fade over time. Then the speaker says "But thy eternal summer shall not fade." So thy (muse) will not fade by being immortalize in "eternal lines." In general, the binaries are life/death, mortal/eternal, mutable/immutable, shining/fading, etc. This sonnet does allude to Heaven and eternal forms, but through the poem itself. The concluding lines sum this up in saying that as long as there are people with eyes to read, the muse in the poem will live forever, uncorrupted and undiminished by the natural course of time.