Three things are being undercut in this poem by Ben Jonson. The first is the lady's right to independent decision. The second is joined in the importance of a good name and the harm of rumor. The third is the nature of a crime. To undercut means to undermine; to diminish in importance; to destroy something's effectiveness; to destroy someone's range of authority or governance. These variations of the definition of "undercut" apply to the lady's right to independently choose and thus independently govern her actions; to the value of moral behavior and the harm of rumor; and to the definition of what constituted a crime in the England of Elizabeth I and of James I.
The lady Celia has evidently rejected the speaker's request for unwed love, "Come my Celia, let us prove, / While we may, the sports of love," and he is responding by attempting to coerce her into agreeing. One of the main arguments behind his disrespectful behavior is that time is not on their side because their good "gifts" of youth "Time will ... sever." He attempts to persuade with an indirect metaphor between youth and the setting sun: a setting sun "may rise again," however when youth sets, fades away, it results in "perpetual night": youth once lost is lost forever.
Another argument is that "fame and rumor" are not as important as they are made out to be. In other words, if Celia loses her moral reputation and good name and if gossips spread (true) rumors about her moral behavior, it really doesn't matter because a good and trustworthy name and a few disparaging rumors are both insignificant "toys" and to be equally disregarded.
These arguments undercut Celia's right to decide without pressure, persuasion, and coercion. The second argument also undercuts the truth of the importance of moral behavior and the harm of rumor, especially if it is based upon truth--which if the speaker has his way, it will be.
His third argument is that they should easily be able to steal away from the prying eyes and overhearing ears of servants and family by their "wile" (clever strategy) and find a remote place to themselves. His fourth argument undercuts the nature of crime in the religious Renaissance period in which they live. He says that stealing love [outside of marriage] is no sin, but that to reveal that love has been stolen, that is the crime:
'Tis no sin love's fruit to steal;
But the sweet theft to reveal.
To be taken, to be seen,
These have crimes accounted been
These arguments undercut Celia's rationality, her right to self-governance, and the nature of society (i.e., right reputation is wrong and rumor is an unimportant toy). It also undercuts the idea of what constitutes crime: in their era, immorality was sin and could be a crime, but the speaker says the reverse.