Eugenics is the "applied science or the bio-social movement which advocates the use of practices aimed at improving the genetic composition of a population", usually referring to the manipulation of human populations.
There is a race, say, the Purple Race.
You separate them into two random gender-balanced groups.
After separation, you tell one group that they must on no account have extramarital sex and God will punish them if they do.
You tell the other group they can have any kind of sex with anyone they like at any age because there is no God and all that matters is that they have fun before they die.
You come back later in 100 years' time to see what has happened to them.
Which group do you think will do better, all things being equal?
Remember: the purpose of marriage is eugenic.