I've always been wondering: why do female voice actors act for male characters in anime?
For example: Mayumi Tanaka for Luffy, Takeuchi Junko for Naruto, even Goku's voice actor (Nozawa Masako), etc.
When there are such good male voice actors (for example the ones for Zoro, Sanji, etc.), why do the main leads get female actors?
Is there any specific reason or is it just the way it is?
Your question is somehow related to this question.
Sometimes, an animated character is better suited for being voiced by a voice actor of the opposite sex. Maybe a higher voice is needed for a male, or a lower voice for a female.
A common variant for this is for young boys, usually 12 and under, to be voiced by an adult woman. This is because real little boys' voices deepen when they go through puberty. Not to mention that it's easier to find experienced actresses than experienced prepubescent male actors. The producers don't have to deal with child labor laws that limit the amount of time a child can spend in a studio, either. Women are often able to maintain the role longer, as well, since their voices don't change like a growing boy's.
There are exceptions, of course — sometimes an adult man or an actual child will voice a little boy. In films, this is the rule rather than the exception, as voice recording for a film generally takes much less time than for a TV series.
And on a related note, sometimes, for comedy purposes, a deep-voiced woman will be played by a man. (source)
Also, female seiyuu voice acting as a male character is much more common than male seiyuu voice acting as a female character probably because it is easier for women to lower their voice's pitch than a man to make his voice's pitch higher like that of a woman.
As to why female dubbers mostly get the main role, aside from the fact that female voices' are more flexible in acting than those of males', popularity also is a major factor. There are many female dubbers that are also in their singing career so it kinda boost up both the anime and the dubber's popularity by having that person get a main role. But I think the flexibility of females' voices are the main reason.
There are multiple cases.
1) Child seiyuu (voice actors) are pretty much unheard of in Japan. (It is not impossible [for example, Matsuura Aya voiced Kaga Rin in Usagi Drop, which aired when she was about 10 years old], but most seiyuu are at least 14 years old when they begin working [such as Maaya Sakamoto when she debuted as Kanzaki Hitomi in Tenkuu no Escaflowne, a character of her own age]). It is common to debut through audition or to complete a degree in an Anime Manga Seiyuu senmongakkou (technical college). Finding a young boy to voice boy roles is uncommon. It is worth noting that very young girl roles are likewise usually played by adult women, not by young girls (for example, the younger sister Mei in Tonari no Totoro). When casting young girls or boys in Japanese theatrical stage productions, usually 2 to 4 children must be cast for a single role, because child labor laws do not allow a child to work more than a certain number of hours per week (for example, Chibiusa and Chibichibi in the Sailor Moon musicals a.k.a. SeraMyu and Rudolf in Elisabeth); this is also partly why it is common to cast adult women as young girls and boys even in Japanese live-action stage plays).
2) Many characters in anime, such as protagonists of extremely long-running children's series, are boys who have not yet hit puberty, which is when their voices would change (for example, Satoshi in Pokemon and Conan in Meitantei Conan). Employing a female seiyuu allows the character a high-pitched, youthful, cute sound, and the series can go on for decades without any need to replace the voice (whereas if a young male had been cast, his voice might break and become too low for the character).
3) Bishounen (pretty-boy) characters are often, though not always, voiced by women. In some cases, the fact that the character is male is not revealed until a number of episodes after the character is introduced, which creates gender-bending hijinks for the other characters who aren't yet in the know.
4) Characters whose sex and/or gender is intended to remain ambiguous are often given female seiyuu (for example, Frol in 11 Nin Iru! and Alluka in HUNTER x HUNTER). Characters who switch sexes are often given female seiyuu, which allows the same seiyuu to voice the character at all times (for example, the Sailor Starlights in Sailor Moon or Dilandau in Tenkuu no Escaflowne). In contrast, in dubs of Sailor Moon in other languages, 2 voice actors were cast for each of the Starlights, 1 to voice the female scenes and another to voice the male scenes. The benefit of the Japanese style is to employ a single seiyuu who can play the entire role as well as perform the songs for CD character image songs.
5) As compared to voice actors in other countries, seiyuu have a very high level of skill and training, and it is a lucrative career. This valid career option allows seiyuu who become famous and beloved by their fans to continue to work for decades (they need not worry about signs of aging as much as live-action film actors do). Other than those with very low masculine voices, their own age in real-life is irrelevant regarding what age of character they can play (this is a reason that child seiyuu are not necessary). Seiyuu can acquire large fan bases, and production companies will hire them partly based on the star power of having their name in the cast (in other words, some viewers will watch an anime simply due to the fact that a certain favorite seiyuu is in the series). Some seiyuu have a wide range of character roles they can play, others are "typecast" --- but in a way that fans know what to expect out of them and it is generally viewed as a positive feature rather than a limitation. As a result, certain female seiyuu are more likely than others to be employed in roles of young boys or bishounen, because they are renowned for such work and/or it's what the fans want to hear.
It is worth noting that sometimes Japan does the unexpected in terms of voice casting. The female mother wolf character of Moro in Mononoke-hime is voiced by Miwa Akihiro, a male seiyuu who is a drag queen with a low, deep voice.
This is common for pre-pubescent male characters who haven't had their voice drop yet. To have people voice male characters like that, the two choices, in general, are having a woman voice them or having a pre-pubescent boy voice them. It's a lot easier to find good good women to voice the boys than to find good young boys.
From Wikipedia:
Voice actors for child roles are sometimes selected from renowned youth theatrical companies, such as the Troupe Himawari. In most cases, adult female voice actors play child roles.
This is also not unique to Japan. Bart Simpson, for example, is voiced by Nancy Cartwright, who also voiced characters such as Chum Chum from Fanboy & Chum Chum. Timmy Turner is voiced by Tara Strong. There are numerous other examples.
