I think something on the lines of Dorothea and Casaubon. Obviously Denethor would be a tremendously charismatic figure: brilliant, those Numenorean good looks, and I think Finduilas would take her role and responsibility as first lady of Gondor incredibly seriously. And I think that Denethor falls for someone young, beautiful, and gentle, who cares for him and is willing to devote herself to him/his job/Gondor. That's how I read it.

And slash young-Denethor stories make perfect sense to me too.