It isn't, any more than having more than one gospel that describes events from a different perspective means that one of them must be "wrong" for the other one to be right (Please note that a lot of what I might seem to advocate is actually just presenting a viewpoint; but not necessarily my own personal viewpoint).
Well, that would suggest that there were other *fully human* beings before Adam, a notion which is in great conflict with biblical literalism
Not really, since the concept itself is a result of being over-literal with the Genesis text. It is, of course, contrary to the most common teachings of the denominations you mention. From some Judaic tradition, there is a belief that there were pre-Adamic men (destroyed in the Flood, which was intended to cleanse the earth not only of sin, but also of the corrupted bloodline of mankind due to intermarriage with Adam's descendants. Of course, that is Jewish myth and not scripture, and I do not present it as such except to mention that it does not explicitly conflict with Scripture).
Well, if one asserts that they are *mere* myths, I certainly could see how you would disbelieve that notion, as a Christian. But what is irrational?
Because of the history of the scripture itself. Rationally (let us pretend for a moment to discount divine inspiration for the purposes of authorship), the one assembling the scripture would reasonably ignore the legends and myths that might provide a viewpoint that contradicts the one held by the assembler. Thus, it seems only reasonable that the assembler of that scripture (historically believed to have been written by Moses, and later restored by Ezra, not that it matters for purposes of this point) would not have been inclinded to provide both stories unless both were accurate reflections of belief. This implies, even in the absence of divine inspiration, that the two stories are fundamentally compatible (of course, that doesn't mean that both stories must be about the same events, just that issues that might seem to contradict are instead misrepresentations or misunderstandings of the text).
Second, casually reading the next few verses, in isolation to Gen 1, seems to suggest that this is when Adam was made (Gen 2.7-8), and only *then* were these plants set in the garden.
Interesting.