Pervert (3.5e Trait)
From D&D Wiki
You constantly find yourself staring at other people's unmentionables.
Benefit: Due to your years of staring at other people's unmentionables, you can accurately determine the gender of most anything you meet - be it undead skeletons, creatures from other planes, or something else entirely. In addition, you get a +1 bonus on search and move silently checks.
Drawback: Nobody likes having a stranger staring at their unmentionables. You take -2 penalty to all charisma based checks when dealing with races that reproduce sexually and that have unmentionables are discernable under normal circumstances.
Roleplaying Ideas: Characters with this trait may be looked on as socially awkward, and some may get angry that the character cannot divert their eyes away from unmentionables.