And a woman can't be referred to as master?
formally, no, they cant be, the proper term in this instance would be "mistress", which is the feminine equivalent of "master".
[edit]
lets see, genders that come to mind with bosses;
skeletron; male (referenced by "master"; masculine term)
EoC; neuter/genderless (deific entities rarely have need for gender as proven by their status, gender becomes only part of pleasure)
BoC; same as EoC for the same reason.
EoW; hermaphrodite; (it is well known that worms are are both genders)
king slime: neuter/genderless/male (the only reference we can be sure of with this, is the masculine title, it is difficult to determine the gender of jello.)
skelly prime; neuter/genderless (this is a mechanical construct powered by souls, the body itself is genderless. refer to fantasy games where animating "constructs" is an option.)
desroyer; same as skelly prime.
twins; same as skelly prime.
duke fishron; male; (duke is strictly a masculine title, female would have been "duchess")
as you can see, we can explore many different points of view and opinion. here are just some of mine