Eh, I really don't want to beat a dead horse and revive that debate again, nor do I have any intention of debating anyone at this late hours. However, I have to agree with Lexifox on this. A retcon is a change to the established canon:
Magnemite changing type is a retcon. Bite changing type is a retcon Move changing power/accuracy is a retcon Lickitung not being able to evolve in early Gen with Rollout is a retcon etc.
But it was never explicitly stated that all Legendary are genderless; it was assumed. That is not canon. All the evidences established is that Mewtwo, Mew, Articuno, Moltres, Zapdos, Suicune, Raikou, Entei, Ho-oh, Lugia, Celebi are genderless, and that fact did not changed in later generations. In Gen 3, genderless species of legendary is still being introduced except for two new legendary species with gender. This is not a retcon but more of a debunking of a fan assumption because nothing has changed as the previous 11 Legendary are still remained genderless. You can't retcon something that didn't physically exist in previous generation.