Author's Commentary:
When I did my research, I discovered that Breyind was right about the secure seat gained by riding astride. Side saddle is very insecure and led to many injuries, especially when a board was sometimes added to keep the woman's feet in place!

And early saddles weren't so well fitted as ours today, so it was easier to share a single saddle then. (The other options found in my research weren't so appropriate - one option was for her to sit behind on a special cushion (probably not very safe) and the other option was for her to literally sit in his lap.)