Here's a thought. The English dialogue we get is a translation. So perhaps what the elves are thinking and saying is more complex and fine-grained, as might reflect the intimacy and mutual knowledge of psychic people who live unforeseeably long lives. So these terms -- lovemate, lifemate, furmate -- are not transliteral terms representing formal romantic institutions that the elves adhere to. They're shorthand approximations, for the reader's benefit, of vocalized concepts that are vague, yet intimately patterned from one partner to the next. Perhaps the elves have 1000 words for lovers, or 1000 timbres or cadences to infuse a word with deep meaning; like Inuit precipitation, all the English reader needs to know is that the snow is soft, hard, or so hard it hurts.