"The Flea" takes place well before the sexual revolution, folks. As in several centuries before. In Donne's day, sex before marriage was consider a serious sin. The woman's parents probably want to see her married to a nice, rich nobleman, not some skeezy poet (sorry, Donne). At the same time, England had a tradition of "courtly love," in which some women took lovers outside of marriage, if they were discreet about it. "The Flea" demonstrates Donne's awareness of both of these attitudes. In the early seventeenth century, a too-free expression of sexuality, and especially a woman's sexuality, could get someone in a lot of hot water.
The speaker stands to lose little from the affair, while the woman could compromise her chances of a successful marriage.
The speaker assumes that the woman is not as concerned as she says she is about any potential loss of honor. She merely wants to make him work for her affections – to "woo" her – in order to fill the conventions of courtship.