From The Westerner Blog
As absurd as this may sound, the sidesaddle took hold in the 14th century to protect the virginity of a teenaged princess traveling across Europe to wed the young King of England. Surprised? Don't feel alone. Most assume the sidesaddle was the natural outcome of fashion, demanded by the long, flowing, sometimes over-hooped skirts favored for so many centuries. But no, protecting the royal hymen was the reason. For some 500 years, women were told the only way a "proper lady" sat on a horse was sideways, holding on for dear life, a passenger on a 1,500-pound animal she could barely control. The fate of that princess makes the story even more ludicrous.
Princess Anne of Bohemia, a predecessor of the modern Czech Republic, was the daughter of the most powerful monarch in Europe in 1382 when she left for England to wed King Richard II. To ensure her virgin marriage, ruling men instructed her to ride aside, rather than astride. "Good Queen Anne," as she'd eventually be called, arrived sitting in a large padded chair, holding onto a pommel in front, both feet resting on a wooden plank that hung on the left side of the animal. (Both men and women mount a horse from the left.) Someone led her horse. She wed a tall, handsome boy she came to love, but who history remembers mainly through William Shakespeare, the playwright who blamed "Richard II" for the Wars of the Roses.
Women hadn't always ridden so askew. Although ancient Greek sculptures depict women riding aside, it was an option, not a demand. Joan of Arc didn't ride into battle in the 1400s as a dainty maid. Geoffrey Chaucer depicted his "Wife of Bath" riding astride in the 1300s. Central Asian women mounted horses like their brothers did, and Amazon women were famous for both their trousers and riding astride. But then came Anne and the fixation on "virtuous virgins." Folks like to mimic royals, so the idea of a sidesaddle spread. By 1600, riding aside was the only way a "decent" woman could ride a horse without scorn. Most women went willingly along-except for Catherine the Great, of course, who was so powerful, she decreed her court would all ride astride.