For me it's an issue of intention. If a game's trying to be as realistic as possible, then of course it's okay. If the game is using the mechanic to give a game sex appeal, then it's just silly. I don't know if it's wrong per se, but it's infantile and meaningless. If that's the intention, then yes, it would be sexist, simply by definition.

As an aside of sorts, this reminds me of the running scene in Juno. Oddly, it's hard to think of the movie's closeup of Michael Cera's bouncing junk as sexist, which is, at least partly, the point I think.

The weight (of history) behind the portrayal of bouncing breasts in the media makes it very difficult to not see bouncing breasts as sexist. The trope is used to define women as sexual objects (and in parallel to define men as perverts).

Just probing here, there's not really an analogue to bouncing breasts, is there? I mean, the bouncing genitals in Juno is about the closest thing I can think of. Perhaps the pitched-tent-boner ala Blink 182's Maybe I'm Just Retarded (I think that's the video) or Ace Ventura 2 or Robin Hood Men in Tights, but interestingly enough, the boner is used almost exclusively in a humorous manner, so what does that say? I'm not being rhetorical because I don't know, really.

I'm trying to think of any game I've played that portrays physically realistic breast movement that doesn't use it just to sexualize the female form. Did you have any particular game in mind, Mirage?