There are also many Male Gardevour, the reason is:
In japan there are people with different ideias, cultures and styles than in another places of the world,
there are many man there who dress and looks like a woman and woman who dress like man, this is common even in many maim-strem japanese culture, rock-bands and games. (the king of fighters is another example)
Then they have the need to please different kind of people, that makes them make a Pokemon who "could" be genderless or have only one genre like many Pokemon who have only one genre, to have both. For example Gallade is only male, but Gardevour could be male or female. She could be only female since it exist only male contepart called Gallade, but they like made them with both gender to please people with different tastes.