A love that was not meant to be. Marisa was destined to be an appetizer dish for a French man and reimu's fate is to have salt poured on her by a very cruel boy, who is also French.
That makes less sense because in nature, salt is a rare commodity and is eagerly acquired by many animal. Same with sugar (carbohydrates). Horses and goats are given salt-licks to balance their diets and they love the taste as well. I'd think it could have an addictive component to it rather than yukkuris hating it.
Bitter and sour things have also been shown to affect yukkuris adversely but i think its more artist/author dependent on what yukkuris hate.