Religion

1. No sex with your relatives.
2. No sex with your mother.
3. No sex with any of your father's other wives.
4. No sex with the daughter of your father or your mother.
5. No sex with your granddaughters.
6. No sex with the daughters of your father's other wives.
7. No sex with your in-laws (two verses cover this, one for your dad's relatives, the other for your mom's)
8. No sex with the wives of your father's brothers.
9. No sex with your daughters-in-law.
10. No sex with your brother's wives.
11. Don't have sex with both a woman (who isn't listed above) AND her daughter.
12. Do not marry the sisters of any of your wives.
13. Don't have sex with a woman on her period.
14. No sex with your neighbor's wives.
15. Don't sacrifice your children to Molech.
16. No gay sex.
17. Don't commit bestiality.

Because we know Leviticus is actually a survival manual, not some weird kinky sex guidebook, we can go down the list and come to one conclusion: The Levites needed all the non-deformed babies they could get, because the infant mortality rate was outrageous. We know now that incestuous relations produce children with massive birth defects and why this happens. So, they made sure to list all the kinds of sex that result in deformed babies. And then we get down to number 15 (because burning our babies at the stake does nothing to solve the population crisis) and 16-17 (because gay sex and bestiality don't produce children at all).