Wow. Ignorant much? The Mosaic laws forbidding close relations were not in effect at the time of Adam and Eve (allegorical or not). No one in their right mind would think that laws put into effect hundreds if not thousands of years after an act would then retroactively condemn someone for breaking a law that did not exist at the time they performed the action.
My personal belief (supported by many scholars, but I'll leave it up to you to take the time to look if you care to do so) is that the Mosaic laws were put into place due to the apparent slow degradation of the species. Adam was said to have lived 900+ years, but by Moses' time, the average lifetime was said to be nearer 175 years of age. Marrying of close relations was no longer a safe option. Marrying a sister or cousin could introduce more and more defects into the human gene pool, so it had to be outlawed. Not that they necessarily understood the intricate details of the genetic code (obviously not), but they understood breeding processes and knew that it caused defects in the animal kingdom, so it's not at all unlikely they understood the consequences of close relations marrying.
So the objection to incest for any rational modern Christian due to the social, genetic, and mental issues it can cause does not preclude the belief that there must have been one initial pairing of humans, and that that pair had children who intermarried to have more children. Both make logical sense.
It is likely we all came from one initial set of 'parents' and that intra-familial marriage had to occur at the beginning. This does not mean that it is fine and dandy for brother and sister to marry each other now, especially when we know better genetically and socially speaking.