While "The Sick Rose" doesn't come right out and say it, it's pretty obvious that it's a poem about sex, at least on some level. A worm penetrating a "bed of crimson joy" sure sounds a lot like sex, as does that strange little phrase "dark secret love." Overall, the vision of sex presented in this poem is dark and even violent.
"The Sick Rose" suggests that when sex and love are paired—when they become the same thing—destructive consequences ensue.
"The Sick Rose" suggests that sexuality should be public, rather than "secret."