Actually, if you re-read Genesis, you'll see that God first offered the animals as partners for Adam. It was only after Adam didn't find any to his liking that God created Eve.
... so, apparently, bestiality has the divine seal of approval, right?
BTW - I can take your argument one of two ways:
1. the argument from design: however God made things, that's the way it should be and it's wrong to interfere with God's design.
2. an argument that Adam and Eve are the model that humanity should follow.
If the first one is what you're going for, then all sorts of things are wrong as well: eyeglasses, airplanes, etc... anything not created directly by God. After all, these are all inventions of humanity that "change the rules" for how things were under "God's design", no?
If the second, then, again, all sorts of things are wrong as well: specialized professions, for instance. I mean, Adam wasn't a mailman, a thoracic surgeon or an evangelist, so it must be immoral to do mail delivery, throacic surgery, or evangelism, right?
Even if they had had more options, the Adam and Eve story could've played out exactly the same if they were both bi.
