It is the nature of love that is perhaps the most important and powerful theme of this poem. Venus, through her infatuation and lust, is driven to pursue Adonis beyond the realms of what is acceptable, and it is Adonis who has to argue with Venus about the nature of love and what love actually means. In his opinion, what Venus is expressing towards him is definitely not love:
Call it not love, for love to heaven is fled
Since sweating lust on earth usurped its name,
Under whose simple semblance he hath fed
Upon fresh beauty, blotting it with blame...
Note how Adoins personifies "sweating lust" as some kind of usurping king who has forced real love to vanish to heaven. The big problem with love, according to Adonis, is the fact that now the "simple semblance" of love has been consumed by lust so that now lust has overtaken love and love cannot be found. In the next stanza, Adonis goes on to compare true love and lust, and lust is definitely found to be wanting:
Love comforteth, like sunshine after rain,
But lust's effect is tempest after sun.
Adonis therefore in this poem is given the position of arguing with Venus that the supposed "love" she feels for him is not actually "love" at all, but rather lust.