Taking up Caroline Lawrence's theme again, I'd like to look at the reasons behind cross dressing in historical fiction. I think this will at least partly answer the point that's come up in the comments; why do we have plenty of girls dressing up as boys but so few boys dressing as girls?
In three of my books, the girls wouldn't dream of dressing up as boys. None of these main characters has any need to step outside her female role, though they might push at the boundaries of what that role allows. But in The Lady in the Tower, Eleanor dresses as a boy once in the story; to take part in a tournament.
This is a very practical necessity. Women weren't permitted to joust, so she needed to be in male disguise. And secondly, you needed to wear armour to do so. With the best will in the world, a Tudor dress wouldn't fit inside a suit of armour. Eleanor has no particular desire to be a man, but she wishes to do something that men do. It's been her dream. So cross dressing is born of a desire to take part in a forbidden acivity.
In my forthcoming novel, The Girl in the Mask, there is a lot more cross-dressing going on. Early Georgian costume for the wealthy (like Tudor dress) was a way of showing status. It proved you could afford a dressmaker, the latest fashions and the costliest of materials. BUT and it's an important but, above all, it proved that you were too weathly to need to work. Because women's gowns were utterly impractical. You were laced tightly so that breathing was difficult; any kind of exertion became problematic. The gown and petticoats were heavy and bulky, so it was difficult to move about. And the fabrics were often brocade silk, satins or velvet so they weren't easily washable. So you couldn't risk playing, climbing, running, sitting on the ground or a hundred other things you might want to do. You could decorate a drawing room or grace a ballroom, flirt with your fan, or if you were very daring, show off your gown at the promenade - which was paved so there was less risk of dirtying your hems.
How can any girl possibly have adventures that are exciting to the modern reader while confined in this way? My character Sophia wants to climb out of her window, roam the city at night, climb out over the city walls and commit highway robbery. She can do none of those things in a brocade silk gown, hoop and layers of petticoats.
Try reversing this. What was there in a woman's world, that a man might be desperate to try out? Um. Absolutely nothing. He could do all of it - more easily - as a man. Nothing was denied to men but fainting and the vapours and they were caused by costume and hardly enviable.
This perhaps explains the lack of cross dressing by men in historical fiction. I'll just mention two exceptions, both from the novels of Georgette Heyer. In The Talisman Ring, Ludovic Lavenham dresses as a French maid to avoid capture by the Bow Street Runners. And Robin Tremain in The Masqueraders becomes a lady to evade capture for his Jacobite past. The former is made comic, as several people have described. But the latter is serious; it is a life-and-death disguise. Even so, Robin is described as 'humilitated' when his disguise is seen through by a friend. But it is only the need for disguise for their own safety that makes this cross dressing even remotely acceptable.
It's not cool for boys to play at being girls. It's not aspirational. Wheras the other way around, cross dressing opens up a world of freedoms and opportunites. As the saying used to go; boys have all the fun. We can be glad that has changed.