When Eve awoke, she felt an intense desire. She was to bear the seed of humanity, but had no male to mate with. Unknowing and uncaring that this would totally mess with God's plan (and humanity for that matter, as Eve would be the only 'pure' human), she set off into the Garden of Eden while God rested.
As Eve wandered the garden, she saw the many species God had created, and how almost all of them besides her, the human, had multiple genders, mostly males and females. She saw so many different kinds of males with different kinds of penises- some of them not even remotely meant to be inside her, but still fascinating. Imagining what it must be like to be their female, to be mated and impregnated by them, triggered her reflexive arousal. For the first time, her vagina became moist and her nipples and clitoris erect, and she secreted feminine pheremones.
One non-human male creature also wandered the garden, rejected by the communities of its own kind and desperate to mate. Suddenly, it picked up on these hormones. Following their trail, it found a strange creature, a tall, smooth being with only a crest of hair upon its head that it realized was alone and also in need of a mate. Something else it slowly came to realize was that mating with this creature would make it unique, and would create offspring of far greater intelligence than those of its own kind.
Eve sensed the creature once it approached her. Spotting it, she was surprised by it, but could sense its own pheremones. They were odd and animal, but what was important was that they were those of a male she could mate with. She smiled and approached it, the mating that would create a whole new cross-breed of species about to occur.