Marcy wears boyish clothing, thinking that this makes her better than girls who wear girly clothing.
Zig Zagged: Sometimes Marcy is seen hanging out with girls and enjoying herself, but sometimes she's seen with men griping about how annoying they are.
Averted: Marcy isn't prejudiced against feminine women.
The writers wanted to do An Aesop on gender equality, and thought the best way to do that would be to make a Strawman in the form of Marcie, who hates women, by virtue of having her change her mind about her gender at the end of her story-line.
The writers want to do An Aesop about how prejudice can come from unlikely places.
Lampshaded: "...What is it with you and other women? Mummy issues or something?"
Defied: "I may be a tomboy, but I see no reason to treat other women badly."
Implied: Marcy is invited to sit with a group of girly girls. and refuses instantly, but leaps at the chance to hang out with a group of three guys. It is never explicitly stated why.
People call Marcy out on her misogyny, and because of it no girls want to hang with her, and the guys she was hanging out with ditch her too, disgusted with her sexist attitude. This leaves Marcy all alone and with no friends, as well as desperately lonely.
Everyone are so tired of Marcy's Double Standard, that they decide to enforce her beliefs to make a point... Starting with Marcy.
Reconstructed: ...that's until she is befriended by misogynistic men and women who hate girly girls just as much as she does.
Played For Laughs: Marcy has to be reminded on several ocassions that she's also female, usually to the sound of a "Oh yeah, I forgot that".
Played For Drama: Marcy brutally bullies anyone who isn't male out of her anti-woman prejudice, and it drives one girl to attempt suicide.