First of all, I would like to thank everyone who commented on my earlier post about the 'button-rule'. I've learnt some interesting things because of your efforts.
For those of you who haven't read their comments, I'll summerize the main points here.
- Thanks to Dora and Vibeke, it is clear that there is indeed a (Old Testament) rule in the Bible, forbidding women to wear men's clothes. As I understand it, it is part of a body of rules covering all aspects of life. Many of these were never obeyed by christians in Europe and some, like this particular one, have been interpreted differently at different points in time.
There might be a semi-religious ground for the 'button-rule' but I haven't found any evidence to support that theory.
- It was Tanitisis who found more information about the theory that women's closures were meant to prevent indecent exposure when riding (or preparing to ride) side-saddle. Apperently, most ladies ride with their legs to the horse's left. I did a quick search myself, and she's absolutely right. However, I don't agree with her conclusion that this would make the buttons gape most visibly.
As this image shows, the opening between the buttons would point to the back of the horse when sitting straight. I guess the stable boy would hold the head of the horse when the lady was getting on it, so this set-up might actually help to preserve her modesty.
Lastly, I can't believe I didn't check Wikipedia on button information before. There wasn't much on the rules for button closures. There was only a short reference to it, in the entry about buttonholes. However, one very important bit of information was added here: according to the person writing this, the maids dressing the ladies, and indeed all common women, would wear shirts which had the same kind of closure as men's shirts. Unfortunately, there's no picture, no specific example is given and there are no dates at all.
If this claim is true, it would be a strong point in favour of the 'ladies being dressed' theory. After all, it is very likely that this feature of ladies' dress would become a bit of a status symbol, which was copied by other women even though it didn't serve a purpose for them. Once adopted by a large percentage of the female population (say, in the Victorian era), it would have become a standard for the growing RTW industry.
I also learned a bit more about buttons and button closures in general. Apperently, the oldest buttons ever found are between 4000 and 5000 years old, made of sea shell and excavated in the Indus valley. There are also examples from Bronze age China and Ancient Rome. In all these early cases, a button was used as an ornament, not as fastening, it was like a sew-on broche.
The earliest functional buttonclosures are from 13th century Germany. They became a common feature on the close-fitting garments fashionable in 13th and 14th century Europe.
Although it's not mentioned specifically in the article, I guess this was mostly in men's wear because, if I remember correctly, women of that era mostly wore wide dresses with separate, laced-up stays (as pointed out in the comments, I'm not correct: stays weren't worn until the 16th century. although lacing was a normal closure on womens' garments). Unfortunately, I don't own a book on fashion history which goes back that far...