Horsey, you are correct about one thing. There IS a gender imbalance in the sport of dressage, just as there is in the other 2 Olympic equestrian disciplines. There are many reasons for the imbalance, but perhaps the biggest one I can think of involves lack of positive media exposure. Dressage, like most horse sports other than racing, receives almost no coverage at ALL in the media and on television. This means that there are few, if any, chances for younger boys and young men to gain exposure to the sport as they grow up, and most people will not emulate or follow idols or heroes who are invisible. Another problem that plagues dressage, and to some extent, the other 2 sports ( eventing and show jumping) has to do with the way the sports are perceived by the non-riding public. Riding in general, unless one is talking about rodeos, is usually considered to be a sport for women and girls, and for the elite, rich, super thin and fit WHITE people of the world. That's unfortunate, and stereotypical, but it's also TRUE, as you have noticed. There's another part of this to think of as well, and that has to do with the fact that most of the men who DO participate in horse sports are usually gay, though there are some noted exceptions to this. Dressage in particular suffers greatly as a sport from this perception that being a male rider means one is gay, because most young men and boys DO NOT want to be labeled as gay, even if they are not, and that alone is more than enough to stop a lot of them from ever pursuing the sport of riding.
Then there is the issue of the attire which people who compete in the sport must wear, particularly on the international levels. Many men I know of will not wear breeches, even those which are designed specifically for them, because they believe it makes them look like they are gay. They are partially right, to a degree- breeches DO have a slimming effect on one's legs and derriere. Even the breeches or jodphurs which some men wear that are part of a military uniform have this effect, and a LOT of men are NOT comfortable with it.
All of this has combined together to create the situation which you now see, and which we in the horse industry have had to deal with for quite some time. In the USA, for every 10 women in horse sports, there is only one man- and that man is usually gay. ( Note that I am NOT talking about racing here- that is a world and law unto itself, and the rules of the game there are totally different.) In Europe, where riding is a respected profession in most countries, it is different. There are LOTS of men in dressage and the other Olympic sports there, and most of the countries there have education systems which allow young men to earn the equivalent of a doctorate degree in horse training, management, breeding, or care.
Can we change this picture? Yes, but it will take a lot of time and hard work to do it. The biggest thing that needs to happen where horse sports are concerned is that we need more media coverage which shows male riders in a positive light. That way, the young boys who are watching might actually get inspired to pursue riding as a sport instead of football or baskeball. The horse world HAS produced some genuine, bona fide heroes and star athletes, many of which are male, but no media coverage means no chance for kids to see them, and it also means that boys who might otherwise want to try the sport get steered into football or basketball instead. I have young nephews, and unfortunately, they are being encouraged to go into football, soccer, basketball, and baseball, because these are the sports they get the most exposure to. Horses to them are nothing but playthings for the rich. That's a perception which needs to change if more men are ever to be a part of these sports which THEY invented so many thousands of years ago.
( Dressage, eventing, and show jumping were ALL invented and developed by MEN, in and through the great armies of Europe and Asia. Dressage is the oldest of the 3- its first principles were laid down by the Greek, Xenophon, who lived and died 400 years before the birth of Christ. Eventing, which was orginally called the Militare, was a sport that evolved out of the tests given to cavalry officers and their mounts, at or near the completion of said officers' and mounts' training. The Militare was like a final exam- and those who couldn't or didn't pass DIDN'T go on to lead combat troops. Show Jumping came about as a result of the jumping contests that senior army officers engaged in during their spare time. Since women were not allowed to serve in the military when these sports were invented, it was natural that there were plenty of men in them in the beginning. )
As for the differences in riding position, those are largely minor, and are usually invisible when riders are sitting on saddles which are designed for them. Some male riders DO have a tendency to sit in what is called a chair seat- and when they do this, it will cost them points, because it is a bad fault. But I have also known and seen plenty of WOMEN who have this same problem, so it really isn't gender specific. Anyone, of either gender, who sits correctly on a horse looks good when the animal moves. That's just my observation- and I used to work for a dressage judge, so I know what I am talking about.