Misgendered Body (3.5e Trait)
From D&D Wiki
You appearance is strongly deceiving in every way, to the point of fooling many, if not all; into believing you are of a gender you are not.
Prerequisite: The character taking this trait must be of a race with actual, separated genders.
Benefit: A character with this trait is born with the physical trait characteristics of a the opposite gender rather than their actual gender (aside from those that would define gender). For example, a female would appear as a handsome or boyish male, whereas a male would appear as a distinctly pretty or beautiful female. The character's voice, appearance and even mannerisms shift to be more in line with the opposing gender. +15 to Disguise checks made to yourself for the opposite gender and +2 on Will Saves on effects that target or affect a specific a gender.
Drawback: +10 to all checks made against you to appear as the opposite gender, and -10 to appear as your actual gender as well as on Disguise checks made on yourself to appear so.
Normal: You appear as a normal specimen of your race and gender.
Special: This trait [can] persists even when a spell, power or like effect forcibly changes your gender or race, such as Alter Self, Polymorph, etc. A character with this trait may not take another trait that would affect their gender, such as Hermaphrodite, Asexual and Androgynous.
Roleplaying Ideas: As a member of your gender it can be difficult to have lived your entire life appearing as a different gender and could have lead to a confusing or hurtful childhood or a troublesome upbringing. Although you do not have certain bodily differences of that gender (breasts, genitalia, etc.), you may have found more acceptance from your opposing gender rather than members of your own. Or perhaps that instead lead to more ostracism and difficulty with attraction to you by members of your same gender. Perhaps a more playful or chaotic character will get a kick out of misleading someone about their gender.