Celebrim
Legend
What if the character is simply a Bishonen?
First, I don't think the discussion of Japanese sexual tropes is one we really want to have since it would certainly derail the thread.
Secondly, in my game at least, an advantage like "being universally loved by both sexes" would not be one you could claim merely by stating that of the character and declaring it in background. No player is allowed to declare tangible mechanical advantages on the basis of the character's flavor or written background. Anything you want to have as a starting that is a tangible advantage must be reflected on the character sheet. If you wanted to be so beautiful that you were universally loved by both sexes, you'd for example take the Attractive and Androgynous traits, a reasonably high charisma and probably some ranks in Diplomacy and/or Bluff. You could then declare your character was so stunningly beautiful that everyone of either sex was fascinated by them, because the mechanical effect would match the flavor you were claiming. You could claim that about yourself precisely because (and to the extent that) it would be true in game that propositions based on that claim would produce the positive fortune outcome.
You are allowed to claim any sort of flavor you want that doesn't imply any direct benefit. For example, you are allowed to claim that you are the king's son, provided you write your background in such a way that you gain no specific benefit from the relationship - for example you are a disowned, disgraced, bastard, who has been barred from court and is neither widely recognized nor widely admired. If you want tangible benefits from this relationship, you must pay for the them with the appropriate traits and feats - noble rank, wealthy, patron, etc.
In general, most DMs without background traits in their game would simply just refuse to approve any background that gave the character considerable advantage above and beyond what could be justified from their character sheet.
I've seen this sort of behavior from a player before. It sucks for everyone, because its blatant spot light stealing and the sort of players that engage in it basically want to play solo with an audience and treat the GM as being a device for self-gratification and self-validation. Any GM that doesn't concede to their demands in every situation is deemed wrong, and so play must stop until they get their way. You end up spending more time arguing about the game than playing it, and if you don't argue, then you must accept that one player in the group has the power of "Win Button" which he may use at any time to get anything wishes.
Rather than the player outright saying "I'm a man disguised as a girl and none of you know", you as DM could simply go with that reasoning unless there is reason to challenge the disguise.
There are plenty of reasons to challenge the disguise. For one thing, the rules on disguise state that you need 1d3x10 minutes to prepare one, and imply that a disguise is in fact a disguise - that is something you apply that obscures your appearance. Disrobing is implicitly equivalent to removing the disguise, particularly in the case of disguising ones gender. For another, if you allow this sort of thing to stand, then you are basically punishing anyone who spent points on being able to disguise themselves. The tacit lesson you are stating at your table is, "Although the rules I've given you to use state that there is a disguise skill that applies in these situations, you are and would be a fool to spend any resources on the ability to disguise oneself because in play I'm going to ignore that in favor of going with what I feel like, up to and including favoritism on my part in how I apply the rules."
Anyone not of the character's race, for example, might have a hard time telling his sex at a casual glance.
While this argument might well apply to an aberration like a beholder or mind flayer, if for now other reason that such races probably don't care about humanoid gender, the same argument seems silly to apply in relation to races with basically human body plans and similar gender norms, such as the fey. Indeed, if anything, fey that might willing engage in sexual activity with humanoids such as satyrs or dryads - as "small gods" of fertility - might well have an easier time determining gender than members of the character's own race.
In many real-life cases, some men just get fooled into thinking a male cross-dresser is a female...
And yet, in general in those cases, the individual in question has gone to great lengths to produce a believable disguise and has great practice in doing so and behaving and speaking like a female and probably spending more even than the usual 1d3 x 10 minutes to produce it. If the PC in question had high ranks in disguise or sufficient charisma, then their ability to create disguises which survived sustained observation might be believable.
go with that assumption until given sufficient evidence to the person's true gender... at which point, in game speak, the man succeeds on his Wisdom/Perception check to penetrate the disguise.
However, the player in question is not submitting to even this most basic concession to the rules. He is declaring by fiat that his disguise is impenetrable regardless of his ranks in disguise (or rather lack thereof) and regardless of the insight and perception abilities of the witness. He's even claiming by fiat that his disguise is impenetrable even in cases where a disguise has not be applied!