This is a fine question. Although John Donne's love poetry is often read as if it is mostly secular (in other words, non-religious), a strong case can be made that most of the love poems are fundamentally Christian in orientation. Some of the poems (such as "The Good Morrow" or "A Valediction: Forbidding Mourning") seem to teach Christian lessons fairly explicitly. Others, however, such as "The Flea" or "To His Mistress: Going to Bed," can be seen as teaching Christian lessons through ironic implication. In other words, some of the love poems seem to celebrate genuinely unselfish, spiritual love of another person by showing such love openly. Other poems, conversely, imply the value of unselfish, spiritual love by showing the absence of such love.
In Donne's day, Christians (who made up the vast majority of the population) were expected to love God first and foremost. If they loved God properly, they would also automatically love everyone and everything else in the universe properly. However, if they loved themselves first and foremost (and were thus guilty of the chief Christian sin of pride), they could never love anyone else or anything else in the proper way. Genuine love of others rooted in love of God was called caritas (charity) and was the ideal kind of love. False love of others rooted in love of self was called cupiditas (cupidity) and was associated with selfish desire and often with mere physical lust.
Thus it is possible to argue that the speakers of "The Flea" and of "To His Mistress: Going to Bed" do not truly love the women they desire; instead, they merely feel selfish physical lust for those women. By mocking such lust through the irony of his poems, Donne (one can argue) actually endorses its opposite, the true spiritual, Godly love known as caritas. In some poems, however, such as "A Valediction: Forbidding Mourning," Donne openly celebrates caritas. He thereby makes clear the positive standard against which he judges the shortcomings of the lust mocked in poems such as "The Flea."