Pastoral literature, as described by literary critic Raymond Williams in The Country and the City, offers an "enameled" or artificial version of the natural world, one that bears little or no resemblance to reality.
Marlowe's poem is pastoral because it provides a picture of living in nature that is at odds with reality. The narrator, fancying himself a "shepherd," weaves a portrait of running off with his beloved to a natural world in which it is always spring ("May"). He tells her that in this idyllic world, "we will all the pleasures prove."
He describes a life of leisure, sitting on rocks watching the other shepherds work. He says he will make his beloved beds of roses and posies, and weave her a gown of the wool pulled from "pretty Lambs." He will make her fur-lined slippers and a belt of "Coral clasps and Amber studs."
This has nothing to do with the life of a real shepherd. The privileged "passionate shepherd" seems to have no idea of the realities of an existence in nature. This is pointed out by Sir Walter Raleigh in his poem "The Nymph's Reply to the Shepherd," written as a response to Marlowe's poem. Here, the shepherd's beloved nymph rejects the shepherd's gifts as false, telling him that "wayward winter" comes and that his ideas are "folly."