Well, the way I think of it, legendaries are genderless, period. Why? Well, they're one-of-a-kind in everything I write. Wouldn't make sense for them to have any 'parts'. And thus, they have no gender at all.
On the other hand, they like to call each other, and be referred to as, either 'he' or 'she'. To some of them it doesn't especially matter which it is. Others will explicitly say they'd rather have one used than the other. Most of the legendaries like to be called 'he', simply because for no particular reason I like to think of most of them as 'male'. In fact... I don't think there are any I especially like to think of as female. =/ Most of the "typically female" legendaries are just genderless in my mind, including Mew, Celebi, Suicune, etc... and a couple even definitely seem male to me for some reason.
If that helps, Frosty, you're not alone: I too always think of Articuno as male. :P