In the English versions of the Gen V games, Zekrom and Reshiram are called the "Deep Black Pokemon" and the "Vast White Pokemon", but in the Japanese versions they're called the "Black Yin Pokemon" and "White Yang Pokemon" respectively.
Yin and Yang symbolize a number of things, but one of them is genders. Yin, the black part, represents female, and Yang, the white part, represents male. Going off of that, one could say that while labeled genderless in the games, Zekrom is female while Reshiram is male. Though from what I've seen, other people tend to call them the opposite, that Zekrom is the masculine one while Reshiram is the feminine one.