This will be explained, expanded in the relatively near future. But yes the Freljord has a very different view of relationships than we do. In part because they live in: a matriarchal society, a historically migratory and often outright nomadic culture, healing magic exists (which reduces infant and mother deaths during birth), dangerous climates, starvation, tribal warfare, and extremely horrific creatures could attack at any time.
So instead being a contract about land and property, the Freljordians live under contracts of obligation and family membership.
Your oath is a statement of fealty to the safety of the person you are under oath to and most importantly their children.
Almost literally, "I swear to die before letting anyone harm you or your family."
In exchange for this pledge you and your family are now a part of that extended network. The oath is also a very public declaration of duty and complicated politically. Because depending on your rank you may have many overlapping commitments, which relate to your social rank within the extended tribe. (So a great male warrior might be Oathbound to the tribal chief (aka a warmother) whom he has no children with, but she might take that man's mate's and his mother's oath as her sisters-- thus their children are now under the tribal chief's protection too. However in that case, his loyalty would be expected to be the warmother and the warmother's children's first- so it's possible the warmother might directly adopt this man's children too in order to more directly honor his importance to her clan. )
All of this ultimately is because mothers and children are considered the most important elements in that society-- esp magically gifted ones. Your future and the future of your clan is unlikely to be determined by owning a castle or farmlands, but it is very likely that having few more children will -esp those who are Iceborn, skinwalkers, or other potential warriors/defenders allied with you.
Masculinity then also reflects these themes. Braum being a prime example. His reputation as a protector of his tribe is more important than who he has defeated in battle or what he has conquered, and this makes him extremely desirable and extremely well respected.
On the woman's side, it would be nearly unthinkable and scandalous for a high ranking woman to have a sex with someone outside of one of her oathbound (which is not to say they have sex with all of their oathbound).
the TLDR is
"I swear to die before letting anyone harm you or your children" + "you are worthy of having sex/ making babies with."
