A - Thanks for the question. There are several possible answers, but the best one is the most obvious.
The best possibility - The stories of Genesis, and the rest of the Bible for that matter, must be read with the culture of the time kept in mind. When Adam and Eve were created we can posit that they had perfect genetics. So, if their children married, they would not have caused had genetic problems for many years. It is only after the gene pool continues to damage itself through the years that it would cause problems in later generations. Thus, the law of Moses says you cannot commit incest (Leviticus 18 and 20), because by this time it would be genetically unhealthy to do so. Thus, by modern moral standards, they must have committed incest. But, because it was the only way to propagate the race, they children of Adam and Eve must have married one another.
Other possibilities have been given, but I believe they are not very good ones. You can read one here.
Pope Pius XII wrote:
When, however, there is question of another conjectural opinion, namely polygenism, the children of the Church by no means enjoy such liberty. For the faithful cannot embrace that opinion which maintains that either after Adam there existed on this earth true men who did not take their origin through natural generation from him as from the first parent of all, or that Adam represents a certain number of first parents. Now it is in no way apparent how such an opinion can be reconciled with that which the sources of revealed truth and the documents of the Teaching Authority of the Church propose with regard to original sin, which proceeds from a sin actually committed by an individual Adam and which, through generation, is passed on to all and is in everyone as his own.I hope this helps.