Sonnet 147 is all about lust but it's not exactly a steamy read. Our "disease[d]" speaker pretty much sums up everything when he says "desire is death" (8).
Gee. Why does he think that sexual desire = death? We're guessing he feels this way because
(1) STD's were a huge problem in Elizabethan England and our speaker may have picked up one of these unpleasant social diseases.
(2) "To die" is common Elizabethan slang for "orgasm."
(3) Some Elizabethan's thought that every male orgasm took one day off the dude's life.
Go to "Themes: Sex" if you want to know more about this.