While we know we all derive our common human nature from this original human couple, the Church does not teach the details of how this worked in first few centuries after them.
We know that Cain and Seth and the other sons of Adam did not just marry their sisters and the human race just naturally continued like that. Even aside from the moral repugnance of such incest, such a genetic bottleneck in the relatively recent past would be evident in the human genome, and that evidence is not there.
We could speculate on a number of other options.
One is that the children of our first parents did incestuously marry each other, but that their genetics were miraculously diversified to avoid the consequences of inbreeding. But this would seem to imply that incest was part of God’s original plan, which is an abominable idea.
Another possibility is that Adam and Eve lived near or even among proto-humans (or whatever they ought to be called) who were virtually identical to us biologically but different spiritually. The sons of Adam and Eve, and any daughters, may have reproduced with these. This idea can strike people as just as repugnant as incest or even more so. It could be seen as bestiality. However we must remember two things.
First, mankind had already fallen by this point and so sinful acts may have occurred without implying divine approval the way the miraculous diversification of genetics seems to. Whatever plan God may have had for later generations of mankind may have been forfeited by Original Sin and what followed instead was a real abomination before the eyes of God.
Second, these proto-humans would have been much more like ourselves than any living animals we are familiar with, to the point that labeling them as brute beasts may be misleading. Perhaps we should follow C.S. Lewis in thinking of them as a sort of elvish folk, in which case marrying them may have been less wrong than we imagine or even, possibly, not wrong at all.
A final theory (though no doubt still others could be imagined) would be that God simply miraculously created wives for the sons of Adam the way He had for Adam himself.