What should adults call the people they love in romantic ways?
If they are married, there's husband, wife or (for the gender equal) spouse. Husband and wife are actually a bit contentious since the roots of the former are related to ownership and the latter are from an Old English word meaning "woman". "Owner and woman"? Oy. So, spouse.
But if they are not married?
Boyfriend and girlfriend seem silly for adults.
Partner sounds like a business thing.
Lover is OK, but ... it could be a one night thing; it doesn't connote a long-term relationship. I like
Inamorata and inamorato (for female and male, respectively)
Definition: These words mean "woman/man with whom one is in love". That's just right.
Origin: According to the ever-useful Online Etymology Dictionary, inamorata and inamorato come from the Latin where it is the past participle of innamorare - meaning "to fall in love".
Why use it? I think these two words: Inamorato and inamorata have tons of uses! I think they are just very good words for a common relationship.