In the beginning, there was but one man, and he was lonely. So God took a rib out of his side and made a woman. So the first woman was a clone of the first man, but with different pieces-parts.
Then the woman talked to a snake, ate an apple, and realized she wasn’t wearing any clothes. She talked the man into biting the apple so he would realize he wasn’t wearing any clothes either.
The Bible doesn’t say whether the first man and the first woman ever bothered to get married. But they must have, otherwise the entire human race was conceived out of wedlock. A short while later, they had a son, then another son. These sons later took themselves wives. That mean the first man and first woman had daughters.
So, the original Biblical marriage means marrying your sister.
Later on. men could take as many wives, and have as many mistresses he wanted. This, too, is Biblical marriage.
If Republicans are so supportive of Biblical marriage, they should repeal laws against incest and polygamy.