Confirm the Fluff with the DM
The only thing important about the exalted feat Nymph's Kiss (BE 44) is what it is (an exalted feat) and what it does (the feat's Benefit). Everything else about that feat is purely descriptive. The Player's Handbook says
Some feats have prerequisites. Your character must have the indicated ability score, class feature, feat, skill, base attack bonus, or other quality designated in order to select or use that feat. ... A character can’t use a feat if he or she has lost a prerequisite. (87)
The feat Nymph's Kiss has no prerequisites.
The DM can always house rule prerequisites for the feat, but if he doesn't, once the player's received the DM's permission and the player's character has taken the feat, the feat's there. Only if the character commits an evil act or is "in areas where magic is suppressed or negated (such as an antimagic field)" (PH 180) does the feat disappear.
As the feat Nymph's Kiss is an exalted feat,
A character who willingly and willfully commits an evil act loses all benefits from all his exalted feats. She regains these benefits if she atones for her violations.... (BE 39)
Thus, if the character were "trying to dupe someone into buying a cursed item" (DMG 277), which is an evil act, all the character's exalted feats' benefits would be lost until he atoned.
Areas of Suppressed or Negated Magic
Strangely, a character loses all exalted feats in areas where magic doesn't function:
These [exalted] feats are... supernatural in nature (rather than being extraordinary abilities, as most feats are). (BE 39)
Luckily, the character with the feat Nymph's Kiss probably spent the skill points he gained, so he needn't subtract those when he enters such an area, but it's something to keep in mind if those feats are prerequisites.
Really, Talk to the DM
The DM must approve the feat, and he must be on board with your character taking a feat that mandates your character be good or lose the feat. If the DM's used to putting devastating moral quandaries in front of the characters, forcing them to constantly choose the lesser of several evils, the character will be hard to play and might not fit the campaign. That's a thing. Talk to the DM. Absolutely.
But only the DM can house rule that your character must do something to keep his feat. That's his call... even if the feat has no prerequisites.