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Found out topic about companions so this already implemented. Now I dont see a necessary to add marriage additionaly.
My suggestion also would be not adding marriage as a lone standing feature, but having npc action and interaction becoming more complex. Like, making the world more alive with autonomously acting beeings, may they be animals or humans.
And at some point, any given individual would perhaps establish some kind of binding to the player's character.
Depending on your interactions, perhaps some simple differences in "npc personality" (like, "naive", "simple-hearted", "trusting", "unprejudiced" "neutral", "skeptical", "wary", "distrustful", "unapproachable"...)
that would automatically implement a kind of life partner/companion, neutral speaking, whatever you make of it.
It could be a woman/man you made gifts to for an exceedingly long time,
or an outcast hunter that you helped with an injury and becoming your friend,
a child you care of after having rescued it from a njerpez village raid
or an old man, who asks you for food and shelter, and gives back some teaching about wooden crafting, cooking, trapping, herbs, or just telling you some stories, because he has nothing else to give...