Cain was in the first generation of children ever born. He, as well as his brothers and sisters, would have received virtually no imperfect genes from Adam or Eve, since the effects of sin and the Curse would have been minimal to start with. In that situation, brother and sister could have married (provided it was one man for one woman, which is what marriage is all about, Matthew 19:4–6) without any potential to produce deformed offspring.
According to this, the "curse" is at it's weakest so incest was ok because it would be free of deformaties. But the way i think about it, is if there was really a "curse" it would be at its strongest when it is put on them and would water down as time went on.....Like if i have black paint and add a little white to it each day it would eventually turn grey. Am i wrong in this thought process? Am i missing something here?