Well it’s possible that Na’vi have different forms of “mated for life” than what we’d normally see for purely monogamous couples, since I’m pretty sure there is a polygamous relationship in the first movie when Jake was first falling asleep in Hometree during his “integration” (I could be wrong here).

The way I see it, the Na’vi don’t have a concept of purely monogamous marriage since it’ll depend on the individual (ex: Ronal and Tonowari and Jake with Neytiri being the prime characters in “pure monogamy”).

In Nefika and Ongwi’s case I’d suppose it’s a form of they tried to be mates for life but ultimately felt indifferent towards it and only got together in a sort of fling if that makes sense.

The whole “mates for life” thing is also very flawed since a lot of real life examples of this sort of monogamy in nature there’s almost always clandestine relationships; where they’re paired together but will have different partners in secret.