What is the theme of "Raleigh Was Right" by William Carlos Williams?

"Raleigh Was Right" was William Carlos Williams's response to a poetic duel between Christopher Marlowe and Sir Walter Raleigh that took place in the late sixteenth century. Marlowe had written "The Passionate Shepherd To His Love," a much-anthologized love poem set against the backdrop of a lush, pastoral idyll and in whose famous opening line a shepherd invites the object of his love, a nymph to

"Come live with me and be my love."

Raleigh's rejoinder was "The Nymph's Reply to the Shepherd." Here, Raleigh brings the world of nature that the shepherd inhabits crashing down to earth. The shepherd's pastoral idyll is really no such thing; like any other part of the everyday world, it is subject to the forces of nature and decay. And the shepherd is also himself a part of that world.
In time, his looks will fade. That being the case, why would the nymph want to stay with him? Of course, if he could somehow retain his youth, then she'd be more than happy to oblige. But that can never happen. The shepherd's mortal world is subject to the ravages of time, whereas the nymph's is timeless, young, forever fresh. Between the two worlds there can be no real interaction.
As the title of his poem clearly states, Williams is unequivocally on Raleigh's side in the matter. I would like to suggest that there are two themes at work here, one explicit, the other implicit. The explicit theme takes its cue from Raleigh's poem. Williams is strongly criticizing the notion that nature somehow provides a refuge from the wants and cares of the everyday world. In this, he is reacting not just against the idealized picture of bucolic life presented by Marlowe but also against the conception of nature associated with the Romantics. Romantic poets such as Wordsworth tended to look upon nature as a living force, possessed with its own unique, awesome spirit that somehow transcended the mundane world of space and time. In doing so, they reified nature; that is, they made it into a thing, something standing over, against, and apart from humanity.
But Williams is having none of this. Nature isn't separate and distinct; the country is as much a part of the world as the city. There's nothing remotely mysterious about it; the gods have long since flown, and with them the simple common folk who plowed the soil, tended the land, and planted the crops. They are dead, and so is the world they grimly inhabited. They, like the shepherd in "The Nymph's Reply," have been destroyed by the violence of time.
That leads me on to the second, more implicit theme of the poem. Williams appears to suggest that the pastoral idyll is no longer a legitimate source of poetic inspiration. Many years ago, perhaps, but not now, not in the modern world. It was all very well in days of yore when the country folk, the common clay, lived off the land. Those were simpler times, a time of "dung and death," to use Eliot's words. It was a time when men and women had a much closer, more intimate connection with the land. It was also a more superstitious time, when every grotto, every vale and tree, seemed possessed by sprites and whispering spirits. Little wonder then that poets waxed lyrical about this animistic fairy world, populating it with long-forgotten pagan deities.
Yes, nature can be beautiful. But it is also an intrinsic part of our world. As such, it can never bring us peace from that world. This applies as much to nature in the here and now as it does to the bucolic fantasies of pastoral poets of old.