Like Meloetta and Virizion, they're clearly female but are stated as genderless. But then we have Tornadus Thundurus and Landorus, and they have genders. Why is this?
It's probably because game freak doesn't want someone to start hating their games because "It's so sexist" and "Well, I think arceus is a girl!" or something. As for the demons of agriculture, I can't say I know about that one. Also, people would want to make game freak cause a new legendary to become the gender of THEIR choice, so it is just easier to say that they don't have genders.