I read an article on Tor.com a while back arguing for more modern relationship dynamics in fantasy. Specifically, the author was asking for more divorce since the honeymoon phase wears off for everyone at some point.
My first thought was along the lines of “oh, goody because I totally didn’t turn to fantasy books for escapism when my dad checked out.” My second thought was more of an anthropological “that probably won’t work.” Marital unions are too important to the traditional fantasy world structure.
Should every fantasy world have female oppression or forced marriage or mandatory chastity? Quite the contrary. However, dumping modern practices into a historical setting with a pinch of magic is not a recipe for an “innovative” or “realistic” fantasy book. Social structures and rules develop in order to solve problems and most fantasy worlds have the same problems as ours did in those periods.
Normalized divorce and sexual liberation in a feudal society where marriage also represents trade agreements and war allegiance is just not possible. Sexual liberation in a world without contraceptives or protection from venereal disease is also really not a good idea. (And don’t you DARE slap a little magic on it and walk away. That’s just cheating.)
Though, to be fair, love matches for royals has never worked at any time in history (arguably even now). But those are still fairly common in fantasy literature. Historically, the middle and lower classes were more liberal, but even they tended to marry for material gain.
All the same, in a world where you are constantly at or potentially at war, you’re going to depend on your family most of all to watch your back. In feudal societies, this has been universally true. That is why a woman’s marital fidelity was so important—to them, knowing who was related was quite literally a matter of life and death. It’s also why even homosexual individuals were expected to have children with heterosexual partners (check out Edward II and basically half the Roman elite).
The only way to reduce the importance of marriage and fidelity would be to have another way of determining one’s allegiance. Maybe all the people who talk to horses belong to one tribe and the ones who talk to falcons belong to another. There are many options.
The point is, cultural taboos (right or wrong) are the results of a culture’s problems. When you solve the taboos, but not the problems, you just end up with a wacky world that is more Wonderland than Westeros. Fantasy authors are meant to have the best literary imaginations, so why can’t they imagine new solutions? Maybe writers should just take more anthropology classes, I don’t know.