"Sonnet 129" takes a rather pessimistic approach to sex and lust. In the poem, the "spirit" which drives man's "lust" is described vividly as "perjured, murderous, bloody," like a criminal whose behavior is driven by madness. Indeed, lust is "savage, extreme, rude, cruel," and absolutely not something that should be trusted. The use of language conjures images of a bestial creature within man and separate from him.
Lust is something that is "hunted" "past reason," until such time as the hunter has achieved his goal, after which he despises his own lust (and, possibly, the object of his lust) as if he has been offered "bait / On purpose laid to make the taker mad." So, after man has expended his lust and regained his reason, he comes to despise whatever caused him to lose his reason in the first place, imagining it as some kind of poison laid out in an appealing way for him to consume.
The concluding couplet aptly summarizes the problem of lust: men know that "this hell" is what will result from pursuing sex, as after they have partaken in it, they "despise" it and themselves, feeling "shame." However, they do not have the wits to shun the "heaven" (love, courtship, sex itself) which will lead them into this misery.