Why were women excluded from the succession line in the Meiji constitution?

_{(I think @congusbongus made some very good points concerning the lack of reasons against male-only succession, but I disagree with the motivations given in that answer. While plausible, "limiting heirs" and "concentrate power" seems to me like deductions borne of faulty premises regarding imperial power. Moreover, the Japanese were extremely concerned with the lack of heirs at the time, with so many of Meiji Emperor's children dying young.)}

The main reason women were excluded was sexism (though not necessarily from framers of the inheritance law themselves - see below). More specifically, Japanese leaders were concerned with (in no particular order):

• Maintaining the lineage of the imperial bloodline
• Possibility of political interference by the consort of an Empress Regnant
• Perception that the consort will be above the Tenno (as husband)

Japan's adoption of Salic inheritance began with the old Imperial Household Law of 1889 (from which the concurrent Meiji Constitution derived its inheritance clause). Its drafting started shortly after the Meiji Restoration in 1868. In most of the early drafts, the traditional eligibility of women was affirmed.

Opposition was however voiced during debates. Notable leaders include the likes of Shimada Saburo (島田三郎), Masuda Kotokoku (益田克徳), Numa Morikazu (沼間守一), and Inoue Kowashi (井上毅). They advanced a number of arguments, which I'll roughly translate below:

Integrity of the royal lineage.

1. 若し然らば人臣にして女帝に配偶し参もらせ、皇太子を挙げ給ふ事ありとも ... 余は畏る、人心(臣?)の血統、皇家に混ずるの疑惑を来たし、為めに其尊厳を害するなきやを。

   If a subject becomes consort to a female Emperor, their union may result in a prince ... What I fear is that when the blood of the subject mingles with the imperial line, the confusion would harm the dignity of the Crown Prince.

   _{- Numa Morikazu (沼間守一)}

2. 男を尚び女を次にするは、現に我国人の脳髄を支配するの思想にして、血統は男統に存すと思惟するも亦我国人の慣性に固着せり。故に此等の点より考ふるも亦女帝を立るの不可なるを知る 。

   Currently, the mindset of extolling men above women is one that dominates the brains of our countrymen. Likewise, patrilineal descent is the sort of thinking our countrymen are used to. Based on this, it takes little thought to realise that a female Emperor is not viable.

   _{- Masuda Kotokoku (益田克徳)}

Interference by the consort

1. 夫の皇婿は政治上の人にあらず、而して暗に女帝の力を借りて政治に干渉す。是れ予がありと云ふ所以なり。何を以て之を言ふ、権力を得んと欲するは人さかんの情なり。而して此情、男子は女子より熾なり 。

   As husband, the imperial consort is not a political position, but may secretly intervene in politics through an Empress Regnant. This would be a malady of the system. A desire for power is a normal human emotion. But this desire is stronger in men than in women.

   _{- Shimada Saburo (島田三郎)}

2. 憲法既に皇帝を政治の最上位に置く。是れ皇婿、陽に女帝の意なりと云ふて、いかん陰に其実力を政治上に施さば、憲法之を如何ともする能はざるべきなり。一憲法あらば百患跡を絶つと思考せるは、余りに事情に迂なるの論とや云はん 。

   The constitution puts the Emperor in the highest position of politics. The consort might claim he is conveying the Empress Regnant's wishes, while secretly applying his own. How could the constitution deal with this? A good constitution should preempt potential problems; from that angle, we should nto complicate things.

   _{- Shimada Saburo (島田三郎)}

Public perceptions of the consort.

1. 我国の現状、男を以て尊しとなし、之を女子の上に位せり。今皇婿を立て、憲法上女帝を第一尊位に置くも、通国の人情は制度を以て之を一朝に変ずる能はざる者なるが故に、女帝の上に一の尊位を占るの人あるが如き想を為すは、日本国人の得て免るゝ能はざる所なるべし、豈皇帝の尊厳を損ずることなきを得んやあに

   The current situation in our nation is that men are held to be superior to women. Even if the Constitution places a female Emperor at the highest position, if she is to take a consort, the whole nation's sentiments cannot be changed in one morning. Therefore the people of the nation would feel that someone occupies an even higher position than the Emperor. We cannot allow the dignity of the Imperial throne to be damaged like this.

   _{- Shimada Saburo (島田三郎)}

2. 我日本現今の社会に於ては、夫婦孰れをか尊しとす。夫に柔順なるを妻の美徳 ... 然るに女帝を立るとせん歟、全国の人皆将さに言はんとす、我陛下は至貴至尊なり、然れども此至貴至尊の御身にし て猶皇婿に柔順ならざるべからずと。是れ余輩が其尊厳に害ありとなす所以なり。

   In the current society of Japan, husbands and wives do not have equal status. Gentle obedience to her husband is a virtue for a wife ... But if a female Emperor accedes to the throne, the whole nation would say, Her Majesty is the most noble and most exalted, but even this most noble and most exalted person would still be obedient to the imperial consort. Then we would have harmed the dignity of the imperial throne.

   _{- Numa Morikazu (沼間守一)}

It was also pointed out that women didn't have the vote, so a Empress Regnant would seem like a contradiction.

凡ソ婦女ハ政権ナキコト一般ノ法ナリ王位ハ政権ノ最高ナル者ナリ婦女ノ選挙権ヲ許サズシテ却テ最高政権ヲ握ルコトヲ許スハ理ノ矛盾セル者ナリ

By law women generally have no rights in politics. The crown is the highest political authority of the land. It is a contradiction in logic that a women who has no right to vote can hold the highest political authority.

_{- Inoue Kowashi (井上毅)}

In conclusion: pretty much all of the arguments basically boil down to sexism (mostly on the part of the Japanese people, or so it was claimed). But note that I'm not saying they, by any means, represent all or perhaps even most of the Japanese leadership. Strong refutations were raised against all of the above arguments by contemporaries.

Moreover, being against limiting the crown to males is not necessarily being any more progressive. For instance, it was suggested that the royal lineage would be preserved with an Empress Regnant, by mandating she marry another member of the imperial family. It fell to Shimada Saburo to rebuff such a measure as inhumane.

Update per Semaphore:
A source for some of the cited statements is the office of the Prime Minister of Japan.