Although he thinks she shouldn't be so worried about the shame of sex, the speaker of "The Flea" lays one heck of a guilt trip on his would-be lover. He stops only a few steps short of, "This is my right as a husband!" (Though he's not, of course, her husband.) As part of his game of seduction, he tries to make her think twice about killing the flea, saying it would be a great crime. But she (smart woman!) doesn't take the bait. She smacks that flea down and, with it, the speaker's arguments. She seems much more worried about the guilt that would be laid upon her by society if she were to give in to him too easily.
The speaker obliquely compares the death of the flea to the death of Christ.
The speaker tries to make the woman feel guilty by treating the flea as a sexual competitor. He acts like a jealous and indignant lover.