Misha and Shizune's relationship is already heavily strained at that point and was never really that healthy to begin with.
Indeed. One of the key points of Shizune's route is that her friendship with Misha is not at *all* healthy, at least from Misha's side of things. A big factor is that of perspective. From Shizune's side of things, she has a friend who came onto her once, but they moved past that and now everything's hunky-dory. From Misha's side of things, she's trapped in the FriendZone - one of the single most painful places a person can ever be - and she has no way to deal with it. She (seemingly) has no one to talk to about the situation except for Shizune herself, and she doesn't dare say anything to her for fear of squicking Shizune out and potentially losing that friendship.
Being stuck in the FriendZone is never a good thing, but it becomes infinitely worse when that person is your *only* friend. You end up in an extremely unhealthy "can't live with, can't live without" situation. Think about it - you're full of sadness and anger and resentment that the person you care about doesn't feel the same way about you. Normally, the thing to do would be to either get over your romantic feelings and move on (healthy), or, failing that, cut ties with the person entirely to at least spare yourself the constant emotional torment (less healthy, but still helps avoid self-destructive feelings).
Misha does neither of these things - she accepts Shizune's friendship even though it's not what she really wants, because it's the closest substitute she can get. Now Misha is trapped in a perpetual 'second place' scenario - one where she *almost* has what she wants, but not quite, and is constantly reminded of that fact. She is, in effect, in denial - taking what emotional scraps she can get and pretending that the situation can survive as it is, not admitting to herself that it can't. As was mentioned before, their present relationship is a dormant bomb just waiting to blow. If Hisao hadn't shown up, the end of the school year would have undoubtedly had the same effect.
The
cheating on Shizune with Misha scene is important from a writing standpoint because it represents Misha's tormented mindset and the way in which she goes about dealing with it. Misha's attempt to
sleep with Hisao is a very unhealthy way to deal with the emotional stress she's feeling - if you agree to it, then you're basically helping Misha to hurt herself. Misha feels even worse about things, Hisao starts to feel miserable too, and that compounded negativity prevents him from helping both Misha and Shizune with their respective problems when they need it most.
Plus, you know, it's also a sort of karmic payback for
cheating on your girlfriend. The lesson is simple, kids -
cheating is considered a bad thing for a *reason*. You're violating a person's trust in you and undermining the emotional intimacy necessary for a relationship to stay strong - you have no one to blame but yourself when it inevitably falls apart.
If, on the other hand, you stop
Misha's advance cold, then it shows how Hisao is dedicated to both of his friends - to Shizune by
not fucking her best friend behind her back, and to Misha by not allowing her to do something that is ultimately emotionally self-destructive. Hisao is strong for both their sakes - he's the pillar of the trifecta that refuses to crumble when both of the others are unsteady, and he uses that strength as the push-off point to help both of the people he cares about deal with their problems in a healthy way.
Hisao came to school in a dark place - depressed, angry, scared, and lonely. Shizune and Misha stepped in and helped him, by giving him friendship, love, and a place to belong - giving him the warmth and stability he needed to recover emotionally. By the end of the scenario, *they're* the ones who now need a helping hand, and your decision at that single, crucial juncture determines whether Hisao uses what he's learned to respond in kind, or throws his newfound happiness away in exchange for
what is probably the most unsatisfying tryst in the history of sex.
To put it simply:
think with your heart, not with your (lower) head.
Besides, why would you *want* to cheat on her? I mean, damn, just [shimmie=2270]*look* at this[/shimmie]. If I was dating this girl, I wouldn't be able to keep my hands off her, much less give a rat's arse about anyone else.
Is there any character that you really hate seeing?
Oh, right...I forgot, there was a thread topic to address in here somewhere.
Well, it's not really *hate*, but every time Kenji pops up, I reflexively say "oh, lord..." in that exasperated way that Leela does on Futurama...