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The basis is that it's a Grass-type pretending to be a bug, as a funny reversal of real-life bugs pretending to be plants (like leaf insects and walking sticks). So it has nothing to with not wanting to give it the Bug-typing out of competitive considerations or something. It's a deliberate...
It's not the sword itself that has a gender, but the ghost who has possessed it. Most other Ghost-types have genders as well, with only a few exceptions like Rotom anad Giratina.
Pachirisu is also in the Field egg group. Plusle and Minun are the only Electric-types who are exclusively in the Fairy egg group.
The others that are exclusive to the Fairy egg group are Clefairy/Jigglypuff/Chansey and their evos, and Audino.
Don't be too quick to overlook Fairy-type Arceus (for which we're likely to get a new Sylph Plate or something like that) and its Fairy-type Judgment attack.