Adam and Eve had other sons and daughters. With nearly a perfect, non-mutated DNA, Cain could have married a sister or niece without a problem of genetic defects. Incest was not forbidden until the time of Moses and the Mosaic law. Abraham, centuries before Moses, married his half-sister and it was not considered incest. There was not enough genetic load (accumulation of genetic defects) for God to have forbidden marriage to a close relative. However, by the time of Moses marrying a close relative became incest.
True, the genetic pool would have been rich and diverse even prior to the Flood. The reduction of the genetic pool at the Flood event to eight people of which only 6 would reproduce afterward, actually is quite well documented by numerous secular genetic studies as a choke point for human genetics. It is also not surprising that the mutation rate of the human genome afterward escalated to the current state of reduced lifespans, introduced more mutations, diseases, etc. Here is John Sanford of the Human Genome Project sharing on the impact of the study on humanity and him personally.