No premarital sex.
No extramarital sex.
No divorce.
Fuck around and find out.
If sex isn't sacred, then sex isn't sacred.
If the people choose sexual disorder, then they get sexual disorder at least.
If the people behave like sexual objects they will be treated as such.
If innocence and impressionability is only found in the preteens, the people can expect hebe/pedo to manifest in their sons.
And if the loss of a blood-deep family member is necessary insurance that a long term relationship isn't just another flakey sexual relationship of convenience, people can be expected to intuit the value in it.
People look for qualities where they are found.
What qualities are found in a minefield of sloppy 300th, provenly flakey divorce and cuckoldry threats with a near 30 year old average age of marriage.
Even sloppy seconds does nothing but degrade marriage and spouse.
This maybe interesting for those who engage with a hefty dose of sexual objectification and shallow expectations.
I have no interest.
So to answer your question:
What qualities can a son find in his mother, for instance.
Devoted, invested, settled, deeply bound, perseverant: a life-long relationship of ups and downs...
This relationship is inherently more difficult to break than some rando.
It's also extremely difficult to initiate for many very good reasons (like compatibility in the first place): and especially the potential corruption or compromising of an existing unique and deeply pure and sacred relationship type.
It's a playful slow burn where even the softest core exhibitionism and tease carries amplified potency.
It's interesting.
Does this seem like it can undergo increases in popularity?
Incestuous reproduction is severely unethical: but this doesn't matter as much with meticulous contraceptive practice and when genetic dead-end sexual relationships are fine.
It's extremely dangerous irl, I don't fully understand the spiritual aspects, and it's probably always a terrible idea.
But even so, sadly I'd rather pretend-fuck my imaginary mother than bother marrying into our critically mismanaged "civilization" of promiscuity and ideological vomit.
tldr;
Pre/Extramarital sex and divorce changes where qualities that people value sexually may be found which serves to explain a rise and fall in popularity of sexual disorder (ie. incest).