The themes of the poem - carpe diem and the immediate gratification of their sexual passions.
Love in the May countryside will be like a return to the Garden of Eden. There is a tradition that our problems are caused by having too many restrictions, by society. If we could get away from these rules, we could return to a prisitine condition of happiness. The "free love" movement of the 1960's was a recent manifestation of this utopian belief. If the nymph would go a-maying with the shepherd, they would have a perfect life.
In quatrains (4 line stanzas) of iambic tetrameter (8 syllables per line, 4 measures per line with 2 syllables in each measure), the shepherd invites his beloved to experience the joys of nature.
He hopes to return with the nymph to a Edenic life of free love in nature.