He married his sister. The prohibition against it didn't take place until Moses.
Genesis plainly says that Adam had other sons and daughters....5:4.
As far as incest? Two things can be said. The human race was propagated from a single pair. This was unavoidable. Also the notion of incest must be probed more closely. At first incest was connected with sexual relationships between parents and children. Only later was the notion of incest extended to sibling relationships.
Moses gave laws governing all forms of incest but this was later down the road. You can research this....Lev 18:7-17, 20:11-12,14,17, 20-21; Deut 22:30, 27:20, 22,23.
We know that Abraham married his half sister, Gen 20:12. So this was not unknown during those times. Moses own father Amram married an aunt, his father's sister Jochebed Ex 6:20. In Egypt the routine marriage of brothers and sisters among the Pharaohs all the way up to the second century made the Mosaic law all the more a radical break from their own past with Egypt.
Genetically reasons for forbidding incest were not always an issue. Back then inbreeding was without serious or genetic damage. Today, the risk is extremely high.
Here's a good link
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