How little that which thou deniest me is; (line 2)
For an unmarried woman in the seventeenth century sex was a big deal. The speaker's goal throughout the poem is to make this big deal seem as tiny as a flea.
It suck'd me first, and now sucks thee,
And in this flea our two bloods mingled be. (lines 3-4)
The mixing of the two bloods inside the flea is seen as being equal to the sex act. We also get the feeling that the speaker is trying to turn the woman on with this image.
Yet this enjoys before it woo,
And pamper'd swells with one blood made of two;
And this, alas ! is more than we would do. (lines 7-9)
Like a child stamping his feet, the speaker is like, "But the flea gets to taste your flesh without having to woo you! Not fair!" The poem itself is the act of wooing. Also, notice that he never speaks directly about sex, as if that might scare her off. Instead, he uses vague language like doing "more."
Where we almost, yea, more than married are. (line 11)
They are "almost" married because their bloods have mixed, but they haven't had, you know, like the whole ceremony, without which you can't be married! Jeez. On the other hand, they are "more than married" because they have already consummated the marriage inside the flea.
Just so much honour, when thou yield'st to me, (line 26)
Not if, but when. You can say one thing about this guy: he's got confidence.
And in this flea our two bloods mingled be. (line 4)
Although it sometimes seems that the speaker is all over the place, this poem actually proceeds in a very logical fashion. He sets up the idea of two bloodlines mixing, symbolizing marriage, within the first four lines.
O stay, three lives in one flea spare,
Where we almost, yea, more than married are. (line 10-11)
The second stanza is directly focused on the idea of marriage. Using that single small word, "yea," as a bridge, the speaker makes the giant leap from "almost" to "more than." From this point on, he no longer even worries about trying to establish that a marriage has taken place; it's a given.
This flea is you and I, and this
Our marriage bed, and marriage temple is. (line 12-13)
Maybe it's not the best idea to compare your marriage to a blood-sucking parasite...or to say that both the wedding and the honeymoon have taken place inside a bug.
Though parents grudge, and you, we're met,
And cloister'd in these living walls of jet. (lines 14-15)
Nobody except the speaker is pleased that the marriage is going forward inside the church, or "cloister," of the flea's body, and it's as if he has tricked her into the wedding. That's the beauty of metaphors; they are sneaky little devils.
And sacrilege, three sins in killing three. (line 18)
Yes, sacrilege is a mortal sin, almost as bad as taking a life (your own or someone else's). In this case, the woman would be committing sacrilege against the holy institution of marriage, the joining of two souls in the eyes of God. But, hey there Donne, if you're so interested in souls, why do you seem so focused on the body, hmm?
And this, alas! is more than we would do. (line 9)
The speaker tries to make the woman feel guilty by expressing feelings of jealousy toward the flea. He conveniently forgets that insects and men play by different rules when it comes to women.
O stay, three lives in one flea spare, (line 10)
When the woman is about to crush the flea, the speaker appeals to her sense of power over it and, in a sense, over him. He's like, "You have the power to spare all of our lives." The subtext is, "If you kill all of us, you'll be a terrible, terrible person."
Though use make you apt to kill me,
Let not to that self-murder added be,
And sacrilege, three sins in killing three. (lines 16-18)
Now the speaker goes straight for the gut, with a guilt-trip that would make our Irish grandmother proud. He's like, "OK, maybe you think killing me wouldn't be such a big deal, but...you'd also be killing yourself and desecrating the sacrament of marriage." He brings religion into the mix, as these are all mortal sins in Christianity. The fate of the woman's everlasting soul depends on her not killing the flea!
Cruel and sudden, hast thou since
Purpled thy nail in blood of innocence? (lines 19-20)
Despite the huge stakes the speaker laid out in the second stanza, the woman kills the flea anyway. He calls her "cruel" and says her action was "sudden" and therefore not well considered. She now has the "blood of innocence" on her hands, as if she had participated in the crucifixion of Jesus Christ himself! These lines mark the high point of the guilt-trip.
Wherein could this flea guilty be,
Except in that drop which it suck'd from thee? (lines 21-22)
Shifting gears a bit, the speaker backs away from the whole, "You're a villainous monster" routine. Instead, he asks the woman to feel pity for the poor dead flea. He's probably hoping these feelings of pity will extend to him, too. He's like, "What did the flea ever do to you?" To which, in perfect hindsight, we might reply, "How about being a carrier for the Black Plague, one of the deadliest diseases in human history!"
Mark but this flea, and mark in this,
How little that which thou deniest me is; (lines 1-2)
For an unmarried woman in Donne's time, reputation was almost everything. If you lost your reputation, you lost all chance of forming a good marriage and might as well enter a nunnery. So in the first lines, the speaker tries to take this enormous source of anxiety and compact it down to the size of a flea.
Thou know'st that this cannot be said
A sin, nor shame, nor loss of maidenhead; (lines 5-6)
The speaker argues that when the same flea bites two people, it is no different from them having sex. If you can't lose your virginity ("maidenhead") from the flea, you can't lose your virginity from having sex. We'd guess that he probably doesn't think the woman is stupid enough to buy this ludicrous argument; he just wants to present himself as an awkward and bumbling charmer, like a Hugh Grant character in a romantic comedy.
This flea is you and I, and this
Our marriage bed, and marriage temple is. (lines 12-13)
Marriage is the height of respectability in Donne's day, so the speaker tries to use the institution to his advantage. He playfully tries to convince the woman that their affair would be sanctioned – nay, expected – by the Church and even by God. They are practically making love inside a temple, for goodness's sake. It doesn't get more respectable than that, right...?
Yet thou triumph'st, and say'st that thou
Find'st not thyself nor me the weaker now. (lines 23-24)
OK, so she doesn't buy the marriage argument. In fact, she literally squishes the whole little marriage-inside-the-flea. But the speaker comes right back at her, saying that if she doesn't feel weaker after having killed the flea, she won't feel weaker after having sex with him. It's another absurd argument, to be sure.
'Tis true; then learn how false fears be;
Just so much honour, when thou yield'st to me,
Will waste, as this flea's death took life from thee. (lines 25-27)
After a long digression, the poem comes full circle back to the idea that having a tryst would not be such a big deal. Once again, the speaker attempts to use the flea's small size to convince the woman that her "honour" (another word for reputation) would not suffer if they did the deed.