Its not only flirting and having free items are the ways you can abuse the sex changer NPC.
Just like what [GM]Bishop said, "The gender-types are saved account-wise. Not character wise. Hence why all characters change at once. "
It is possible that it may cause additional lags and bugs in the server if many of us, players would have change sex. I think that 10 tokens would be enough to control the demand of having changing sex.