Leviticus 18

Leviticus 18 covers unlawful sexual relationships. This covers the gambit from humorous to just plain gross. Many of these are undoubtedly considered "incest" in our modern culture and are heavily frowned upon today. A couple I want to highlight:

No sex with your half-sister (v.11), daughter-in-law (v. 15), brother's wife (v. 16), wife's sister as a rival wife (v. 18). We saw all of these situations in Genesis with the Patriarchs. Abraham were Sarah were half-siblings, the Tamar and Judah incident involved a brother's wife and daughter in law. Finally, the rival sister wives reminds us of Leah and Rachel, wives of Jacob.

None of these were ideal situations and the relationships caused a lot of problems.

However, verse 9 forbids sex with your sister. Gross. But this would cause problems if one reads Adam and Eve to be the only humans on earth that were directly created by God. Then, all their future offspring would be forced to violate these rules. Likewise, they were commanded by God to be fruitful and multiply, so God is requiring them to break this rule.

A possible resolution could be found in the beginning of Lev. 18, where God explains these rules as being a separate practice from what they do in Canaan. Likewise, intermixed with all the rules on forbidden sex a requirement to not offer your children to the god Molek (v. 21). This would suggest these rules are to distinguish the Israelites from the worship of Molek.

That said, Lev. 18 concludes by saying that these practices defile the land and the land will vomit you out if you do them. This creates a quandary as the land did not vomit out the Patriarchs for doing this, while in contrast, Adam and Eve's offspring were implicitly instructed to break these rules.

I don't have an answer to this.