It's very common that female seiyuus play boys or men, rarely male seiyuus play women. However, even the Pretty Cure has their very "boyish sides" (joke).
Some examples. I will actualize it.
Cure Dream[]
Seiyuu: Sanbei Yuko
Renton Thurton from Eureka Seven.
Pride/Selim Bradley from Fullmetal Alchemist Brotherhood.
Alexander "Sasha" Nikolaevich Hell from Seikon no Qwaser.
Fuuta from Katekyo Hitman Reborn!.
Cure Rouge[]
Seiyuu: Takeuchi Junko
She is famous for playing boys.
Uzumaki Naruto from Naruto.
Kanbara Takuya from Digimon Frontier.
Dieter from Monster. ("Dankeschön.")
Kaiba Mokuba from Yu-Gi-Oh! Duel Monsters (2000).
Lambo (child) from Katekyo Hitman Reborn!.
Gon Freecss from Hunter X Hunter (1999).
Cure Lemonade[]
Seiyuu: Ise Mariya
Killua Zoldyck from Hunter X Hunter (2011).
Romeo Conbolt from Fairy Tail.
Cure Berry[]
Seiyuu: Kitamura Eri
Yasaka Mahiro from Haiyore! Nyaruko-san.
Cure Blossom[]
Seiyuu: Mizuki Nana
Wrath from Fullmetal Alchemist (2003).
Cure Sunshine[]
Seiyuu: Kuwashima Houko
Kyouju/Saien Manabu (aka Kenny) from Beyblade.
Cure Moonlight[]
Seiyuu: Hisakawa Aya
Sohma Yuki from Fruits Basket.
Rei Kon from Beyblade.
Kero-chan from Card Captor Sakura.
Cure Melody[]
Seiyuu: Koshimizu Ami
Hirose Takuma from H20: Footprints in the Sand.
Cure March[]
Seiyuu: Inoue Marina
Erio Mondial from Magical Girl Lyrical Nanoha StrikerS.
Seno Natsuru from Kämpfer.
Cure Diamond[]
Seiyuu: Kotobuki Minako
Rickert from Berserk: The Golden Age.