I dunno, I prefer them as boys. While it might be silly with this cast to say that too many girls would risk monotony, I think it's perfectly reasonable to say that having sons among the daughters gives the story a different texture.
The mentality to want just one sex of offspring (900% of the time IRL, boys) is what has so horrifically skewed the sex ratios of India and China, ironically also the world's two most populous countries. You could say, the fans of Touhou who draw these comics are "getting back" at the real world by making more daughters than sons, though there may be other reasons as well.
