To the right is a side saddle from the 17th century. It is clearly designed to limit a woman’s ability to ride athletically–more of a way to carry her from one place to another at a walk, then as a sensible mode of transport.
This saddle is, however, a later invention. In England (and Wales) it appears that women in the Middle Ages rode astride much of the time, either on their own or pillion behind a man. Here are two pictures of women riding: The Prioress from The Canterbury Tales (dated to 1532) rides aside and The Wife of Bath (1410) rides astride (the Ellesmere Manuscript, c. the Huntington Library, San Marino, CA). For more discussion http://ilaria.veltri.tripod.com/sidesaddle.html
Riding astride is certainly far more practical, provided the dress or gown a woman wore could be arranged modestly. Given the voluminous skirts that women often wore, that was perhaps not as difficult as it might appear. Victorian women, in contrast, wore much narrower skirts and rode aside–it would have been impossible to ride astride in them (probably part of the point) without hiking it to the waist or ‘kilting’ it, as Brother Cadfael does with his monk’s robe.
At the same time, riding aside is a very old custom, all over the world. There are images on Greek Vases of women riding aside: http://www.theoi.com/image/P12.4BThetis.jpg
As well as on Celtic stones, namely the goddess Epona: http://www.epona.net/depictions/strasbourg.jpg
It is odd, then, that “history credits Anne of Bohemia (1366-1394) with introducing the earliest version of a functional sidesaddle, a crude chair-like affair with a planchette (small foot rest) her design no doubt inspired by having to ride swaddled in elaborate gowns.” http://www.mtholyoke.edu/courses/hgarrett/medst300/forum/messages/78.html
Perhaps it is, rather, that she (re) introduced the side saddle to Europe.