It’s really important that you know the paternity of your children because we live in a society where all things past to the first born sons, and all titles, properties and holding accumulated through tithing your people and reaping the crops of your fields must travel down through blood lines.
We aren’t that far from feudalism in a lot of ways.
Sexual openness makes it harder for men to control women, practically and generally too. If women are open, and allowed to have sex with whom they will, then men can’t protect their eggs from stimulation from the wrong sperm. There’s doubt. Doubt about bloodlines.
But in this day and age, does that really truly matter? It’s a provocative thought, I’ll grant you, but what’s the actual function of bloodlines if you’re committed to raising your partners kids with her. Think how many examples there are of folks raising their children without the children being genetically similar to one of the parents. Does it make any inherent difference in that child’s care and raising? I’ll wager that it does not.
There’s so much good that sex positivity and openness can bring to a family, and to people individually, and so very little to lose, and yet it’s one of the most important mores that people hold on to. That will change. But it will also always be very similar percentages as it is today.