Mourning Wood's a guy if you know what I mean.
My comments in yellow.The King Slime is a boy since the ninja inside him is a boy and he is a king. Because you eat a boy doesn't make you a boy. but right on the king though.
The Eye of Cthulhu is a boy since he is the eye of the mythical beast Cthulhu. Eyes can't have a gender
The Eater of Worlds is a girl since she lays eggs. (Shadow Orbs) interesting found, honestly.
The Brain of Cthulhu is a boy, he is the brain of Cthulhu. Brains can't have a gender.
The Skeletron is a boy, the clothier is a boy and skeletron cursed him right. just because you cursed a male doesn't make you a male.
The Queen Bee, obviously a girl right.
The Wall of Flesh is a boy since the Guide is slain from the lava in hell and he has to be sacrificed. The WoF is a boy because he is the Guide himself. Depends on theory.
The 3 mechanical bosses are robots, no gender. They might be Cyborgs of the previous bosses, once again, depending on theory.
Plantera is a girl, plant, pink, big . She is a girl. You should probably look at the plant's genetials instead of looking at the color.
Golem is kind of a mechanical boss so no gender.
The lunatic cultist is a boy. obviously. Is it obviously? Are there any reasons why it couldn't be a girl?
The moon lord is a boy since he is the brother of Cthulhu and he is a spirit.
Duke Fishron, he is named DUKE Fishon, can't get easier than that.
Ocram is a boy, I dunno why I just say it. Likely because it's name reversed makes it Marco.
Calling ships "she" is a long tradition, and it seems the Flying Dutchman is no exception despite her manly name. (Some quotes here.) So you definitely have a good point there.You know how people like to give their cars names and call it a "she" i think the same sort of logic can apply to the flying Dutchman so it's a she at least in the eyes (well eye) of the captain.