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Reshiram's body looks fluffier and more effeminate, and even appears to be wearing a bra. Zekrom's body looks harder and more masculine, and has visible muscles.
But the white yang represents maleness and the black yin represents femaleness, and Reshiram clearly has a dick tuft and Zekrom has a slit down the middle of its crotch.
From this, I conclude that Reshiram is a femboy, and Zekrom is a tomboy.