And the art, I must add, is beautiful. Just... Amazing.
Well, like I said, we have our own preferences.
I know they're genderless, but I imagine them acting as if they were a certain gender.
I have found the setting of them on pokemon wiki, the gender of them is "ふめい (不明)" in japanese and it means "unknown".
The explanation of the difference between ふめい pokemon and other pokemon is "cannot breed with any Pokemon other than Ditto and they cannot have any Egg moves" and mentioned "machine, the inanimate object, genderless pokemon, and the legendary Pokemon is ふめい ", but never mentioned "genderless pokemon = legendary Pokemon".
So I think "genderless" is only a translation errors
So, in this couple, Manaphy and Celebi, who is what gender?