Very soon in the second part of the sonnet cycle, the reader discovers that the "love" that the speaker feels for the Dark Lady is a far different kind of emotion than the love that he feels for the Young Man. In the third sonnet -- only the third sonnet -- in this section, the speaker moves away from praising and adoring the lady to the topic of lust. Lust, you may recall, is one of the Seven Deadly Sins that all Christians during the Middle Ages would try to avoid. In Shakespeare's poem the speaker worries not about what may happen to his soul in the afterlife. Rather, he worries about what spiritual (or, perhaps, psychological) damage his lust is causing him during his own earthly existence. "Sonnet 129," then, is a warning about the dangers of lust.
In terms of structure, the division in this poem comes between the three quatrains and the couplet. In the first three quatrains, the speaker defines lust and explains the effects that it has on humans. In the final couplet he adds one additional problem that is associated with lust.
In the first quatrain the speaker begins by explaining that when a man acts on his lust ("lust in action") and has a strictly sexual relationship, that man is wasting his spirit in an act of shame. However, if a man does not act upon his lust ("till action"), then that man is capable of many dangerous actions: perjury, murder, violence, cruelty, or deceit. The speaker is implying that man is capable of the