Then worms shall try
That long preserv'd virginity,
Holy men would teach lusty youths to kill their sexy thoughts by thinking about the object of their affections as a corpse getting eaten by worms.
Here, Marvell invokes an image of worm-eaten flesh to get the girl excited, because nothing turns girls on more than vividly imagining their own physical decay. (Marvell has weird game, but so does Maboo).
You could also read this as Marvell threatening the young woman: “Either you lose it to me now, or you’ll lose it to maggots in the grave!”
More charitably, he’s following his previous line of thought. It’s wasteful for her to continue to refuse his advances. Time flies. One day she’ll be dead. Why preserve her virginity for the worms when he would happily take it now. He wants her to live in the moment.
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