This broad leather band appears to be a normal belt used commonly by all sorts of adventurers,
but of course it is magical

.

If buckled on,
it will immediately change the sex of its wearer to the opposite gender.
Its magical curse fulfilled,
the belt then loses all power.
The original sex of the character cannot be restored by any normal means,
although a wish might do so (50% chance), and a powerful being can alter the situation,
i.e., it takes a god-like creature to set matters aright with certainty.

<Girdle of Noninity> : 10% of these girdles actually remove all sex from the wearer.

<titans, from the DDG, not the MM, can be considered 'god-like' creatures : RFI.11><check this>