anime that has a main charater sex change?

Hourou Musuko.

Otome Wa Boku Ni Koishiteru
http://en.wikipedia.org/wiki/Otome_wa_Boku_ni_Koishiteru

Miyanokouji Mizuho has to cross dress as a girl in order to attend Seio Girl's Academy per a request in his grandfather's will.

Oh My Goddess does this for one episode, but it's not really a part of the story. Not that OMG has much of a story, it's mostly just random comedy.

There's a show called Birdy the Mighty Decode that does something similar, not a traditional gender bender but with the same paradigm. I haven't seen it myself, but those who have watched it seem to think it's decent.

I My Me Strawberry Eggs
Princess Princess
Maria Holic
There is one episode of Black Butler where Ciel has to be dressed as a girl.
Ouran High School Host Club (girl character into a boy the whole anime) But there is an episode where all the guys dress as girls. And Haruhi's dad is a cross dresser.
In Lovely Complex there is a guy that dresses as a girl but he isn't the main character. The best would be Princess Princess or I My Me Strawberry Eggs or Maria Holic

sry i dont think there are any besides rama 1/2 which is rly good!
i think u should still try it out even if its old.