A while ago now, someone started a discussion on the Sailor Moon Fans LJ about a scene in S - the one in the top three pictures, when Michiru and Haruka are first introduced. Minako ask Michiru, “Are you Haruka’s lover?” with Michiru replying “no”, and the person was interested in why. My theory is kind of strengthened by this later scene.
For starters, in that scene, the question wasn’t quite so straight-forward. Usagi added the restriction, “Please answer ‘yes’ or ‘no’!”. So, it’s different from Michiru just flatly denying they were in a relationship; she was denying that they were lovers. When given a choice between “yes” or “no”, “yes” means that they are only lovers, but “no” is the only one that can mean “we’re much more”.
In the next scene from a later episode, if you’re not familiar with it/have forgotten, Haruka and Michiru have entered a gaudy “true love” sort of contest/live game show (I’m not sure why; possibly they are vampires who feed off others’ imperfection). In this part of the game, the men (“men”) were asked to identify their partner from only their hands. When Haruka chooses Michiru’s hand, the announcer asks her to shout out her lover’s name, to which Haruka takes offense. She’s not trying to deny that they’re a couple so much as she is objecting to the idea that her relationship with Michiru can be defined by something as mundane as “lovers”. The word in Japanese - koibito - just means “person you love”, and it’s not as intimate as the English term “lover” is; more comparable to boyfriend/girlfriend/sweetheart/etc. I think Michiru and Haruka believe that they are fated to be together, linked by destiny, that their relationship is unique and special and goes much much beyond just “this is the person I love”, and becomes “this is the person whom I am destined to be with, by fate and duty, in our past life and our next”. So I think both of them laugh internally at the idea of describing themselves as “lovers”.
Anyway, so that’s why I think Michiru said ‘no’.