John quotes from act 3, scene 3 of Romeo and Juliet, in which Romeo has been banished from Verona and is complaining to Friar Laurence that even animals—even flies—will be allowed to touch Juliet, while he will not because he will be so far away. Romeo says of the flies that they
On the white wonder of dear Juliet's hand, may seize
And steal immortal blessing from her lips,
Who, even in pure and vestal modesty,
Still blush, as thinking their own kisses sin.
This quote springs to his mind when John gazes down on Lenina as a fly buzzes around her. He has crept into her rooms while Bernard is away in Santa Fe, and while Lenina is off on a soma-induced drug trip.
John uses these lines because Shakespeare is what he knows and loves, and a Shakespearean cultural context structures his thinking. The lines foreshadow the cultural dissonance between John and the World State, as well as the disconnects that will destroy any chance of John and Lenina having a successfully relationship.
But, primarily, Huxley is using the lines at this point to comic effect. Like Romeo, John is completely lovesick, but unlike Romeo, has chosen a shallow love object unworthy of his adoration. If John had the least inkling of the way Lenina has been raised, such as to give herself sexually to every alpha man she comes in contact with, he would never see her in terms of "pure and vestal modesty" or imagine flies blushing and thinking of their kisses of her as "sin." She has no idea of "sin" and certainly would think it the height of social and mental unbalance to expect to be "pure" or modest.
Huxley uses a favorite device of Shakespeare in this scene: dramatic irony. We know the values of the World State and how completely different they are in terms of sexual practices from John's traditional worldview. John, despite his own mother's behavior, does not really have an inkling of the mores in the World State and casts Lenina in a laughably wrong light when he thinks of her as chaste and pure.
The Shakespearean context provides a further irony. Romeo is mourning being banished, and John expects to soon be separated in a like way from Lenina, as he knows she and Bernard will soon leave the Savage Reservation. What he doesn't know—and we do—is that Bernard is in Santa Fe arranging for he and Linda to come back to the World State with them.