But this seems wrong. It seems that a world in which a female member of *NSYNC existed is POSSIBLE, and therefore "All members of *NSYNC are males" is only a contingent truth.
Well, you seem to have already determined that this specific human invention falls within white crow
territory. Unless there is fundamental reason why it is impossible for females to be members of a specific organization, such as it being a male escort service (which would then contradict its very definition). Even there, though, it depends upon the constraints of "membership" -- whether or not it refers purely to the escorts or also includes non-escort working units that contribute to the viability of the operation. Trans-gender and post-op members are excluded or included according to however strict the meaning of "human male" is regulated during the era (or narrowly defined by the organization).
Concerning the broader issue of "necessary truths": Supposedly there will be a need to distinguish unmarried human men -- in any domain where they arise -- with a concept accidentally represented in English by the word "bachelor". Supposedly in any realm where distinction and division is the case, cognitive agents therein would need to devise a quantitative system that provides abstract placeholders for the particular instances of these individuations, where they are both distintegrated from and integrated into groups or sets -- and 1+1=2
would thus be a rule inexorably applicable in all of them. And so forth.
That is, some "necessary truths" actually were extracted originally from concrete circumstances before their conversion to abstract generalizations. While other "globals" may be contended to emerge unavoidably from the basic elements and maxims of an invented system as it develops in complexity, without any correspondence to circumstances of the observed world (at least in the beginning). Yet these higher-level necessities supposedly applying also in any world where the same system is viable. (Invented schemes can have their immutable standards, despite whatever controversy absolutes have in a scheme-independent, "encountered reality over time").