I do not recall it ever saying there could not be any female Knights? Been decades since broke out the books.
The DC comics had a woman on the road to becoming a Knight. I followed the DC D&D lines for a good while. Some of it is pretty good too. Some is kinda meh. For some reason the Spelljammer one felt the most disappointing. Like it was missing the spark of the older series and seemed a little disjointed.
Good question, and good point. I believe you could play a female knight in the Gold Box games. I do not recall a specific rule against them being documented anywhere, however, it seemed strongly implied.
It would be interesting if anyone here can find an example of a female Knight in the Age of Despair or earlier.
Some items that I believe to be true, but please check my facts:
1. Vinas Solamnus founded the knights, all of them were male. Luccia Solamnus was his wife and childhood friend. They joined the army together and fought in battles for a considerable amount of time. This predated the founding of the knights. She was never listed as one of the founding knights after being formed by Vinas. Their son was a founding member.
2. This is from memory, but I do not recall any mention of a female in the knighthood in the Legend of Huma. This significantly predates the War of the Lance, but speaks to the pattern.
3.
There is no mention of females in the knighthood in the original trilogies. In fact, they make a not so insignificant deal about Laurana leading the knights as the Golden General. In fairness, she was elven and not from Solamnia (being of Solamnia is 100% a requirement), but there were references to her being a woman as well from what I recall.
Edited: Armchair proved this to be wrong!