I know this may sound silly, but the GOP's fanatical campaign to define marriage as "between one man and one woman" does not leave out the prospect of marrying your sister/brother or first cousin. I don't see any Christian or Republican posters that read "Marriage is Between One Man and One Woman Provided They Are Not Your Sibling or First Cousin"
If their so-called "marriage amendment" passes and becomes part of the Constitution, will it prohibit marriage with your brother/sister and first cousin? Will it include the passage from The Bible forbidding interrelational marriage:
The more distantly related parents are, the more likely it is that they will have different mistakes in their genes. Children, inheriting one set of genes from each parent, are likely to end up with pairs of genes containing a maximum of one bad gene in each pair. The good gene tends to override the bad so that a deformity (a serious one, anyway) does not occur. Instead of having totally deformed ears, for instance, a person may only have crooked ones! (Overall, though, the human race is slowly degenerating as mistakes accumulate, generation after generation.)
However, the more closely related two people are, the more likely it is that they will have similar mistakes in their genes, since these have been inherited from the same parents. Therefore, a brother and a sister are more likely to have similar mistakes in their genes. A child of a union between such siblings could inherit the same bad gene on the same gene pair from both, resulting in two bad copies of the gene and serious defects.
Adam and Eve did not have accumulated genetic mistakes. When the first two people were created, they were physically perfect. Everything God made was "very good" (Genesis 1:31), so their genes were perfect -- no mistakes! But, when sin entered the world (because of Adam -- Genesis 3:6, Romans 5:12), God cursed the world so that the perfect creation then began to degenerate, that is, suffer death and decay (Romans 8:22). Over thousands of years, this degeneration has produced all sorts of genetic mistakes in living things.
Cain was in the first generation of children ever born. He (as well as his brothers and sisters) would have 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 (it takes time for these copying errors to accumulate). In that situation, brother and sister could have married with God's approval, without any potential to produce deformed offspring.
By the time of Moses (a few thousand years later), degenerative mistakes would have built up in the human race to such an extent that it was necessary for God to forbid brother-sister (and close relative) marriage (Leviticus 18-20).(Also, there were plenty of people on the earth by then, and there was no reason for close relations to marry.)
http://www.christiananswers.net/q-aig/aig-c004.html
Don't the Repubs see how ridiculous this is?