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.