| Quote #1
Mark but this flea, and mark in this,
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.
| Quote #2
Thou know'st that this cannot be said
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.
| Quote #3
This flea is you and I, and this
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...?