I wouldn't say that John Donne's "The Flea" expresses the theme of love so much as sexual desire. In that regard, Donne was able to draw on an existing convention for the central conceit of his poem. It was quite common in those days for sex to be represented as the mingling of blood. And the flea's nature as a bloodsucking creature makes it a particularly suggestive metaphor to use in relation to such a risqué theme.

Beneath all the ribald humor and innuendo, Donne's message to his lady is a simple one: he wants to have sex with her. Whether there's any actual love involved here is a moot point. What matters to the speaker more than anything else is that he and his lady should become as one, i.e. join together in the act of sex.

The blood of the speaker and his beloved have already been mingled together in the body of the flea, who's bitten both of them and sucked their blood. In Donne's eyes this makes him and the object of his desire almost "more than married". This particular line from the poem comes close to suggesting that there's more than just sexual desire going on here; that perhaps the speaker seeks a spiritual, more meaningful relationship to his lady.

But the theme of sexual desire soon returns with a vengeance, with the lady squashing the flea with her nail, thus destroying, in metaphorical terms, the symbol of the speaker's desire for her. It's clear from this "Cruel and sudden" action that she doesn't see their relationship in quite the same way as he does.

The answer above gives some very good arguments as to why “The Flea” does not convey the theme of love. While these are all very valid, I will attempt to provide an alternative perspective.

Donne belonged to the Metaphysical school of poetry; he and his fellows were concerned with wordplay and conceits, and in approaching common subjects in uncommon ways. In writing a love poem to his mistress that so blatantly expresses a desire to mingle their blood (note that it was believed this happened during impregnation) Donne may be satirizing or critiquing other love poems, which pretend to have no interest in sex. Donne is not expressing sex without love, but simply acknowledging that love encompasses sex; it is not a lofty and unearthly thing as much love poetry presented it.

We do see language pertaining to marriage repeated throughout the poem. Donne is not simply attempting to seduce a mistress in whom he has no lasting interest. Instead, language such as “cloistered,” “sacrilege” and “marriage temple” suggest quasi-religious elements, and certainly a sense of longevity. Donne expresses a view that sexual intercourse should not be seen as a sin, because it is no more or less intimate than two people being bitten by the same fly.

He is sure that his beloved will “yield” eventually; arrogant language, certainly, but Donne seems to indicate that he feels marriage should not change the nature of a true relationship—not that the relationship depicted is at all meaningless or shallow in his mind.

In John Donne's "The Flea," the speaker is not expressing his love for the woman to which he speaks, but his lust: for his argument about the flea is his attempt to convince her to sleep with him, a plea that she has been resisting.

The speaker argues that as the flea has bitten both of them, their blood is mingled and they are more closely tied, as they would be should they go to bed.

It sucked me first, and now sucks thee,

And in this flea our two bloods mingled be...

He goes on to assert that in joining their blood, nothing shameful has taken place: she has not slept with him, lost her virginity, or done anything that would upset her parents. But he notes that the flea did as it pleased with her, without having to win her, as the speaker is trying to do. He explains, then, that they are joined in the blood of the flea, and this is something of a "marriage bed." And he reminds her not to kill the flea, or she will be killing not only the flea, but also herself and the speaker...at least in terms of this unusual union.

He asks her:

Cruel and sudden, hast thou since

Purpled thy nail, in blood of innocence?

Has she squashed the flea?, he wonders. For if she has, she will have murdered an innocent victim, for the flea was not a guilty creature. One may assume that the argument falls flat on its face, in that it does not convince the lady to acquiesce.

Her quick response with her nail in the face of this man’s elaborate plea suggests her response to his final assertion as well.

There is no mention of the speaker's love, abiding or gentle: he simply argues why they should have sex. This is not a poem of love, but of seduction by logic as the speaker tries to give irrefutable evidence to convince her to stop her resistance to his advances and give in.

(This is a poem of Donne's earlier years, when he was known "as a frequenter of ladies," before he married, changed his behavior, and became "the dean of St. Paul's Church." This further proves Donne's less than noble pursuit of the woman in the poem.)