I think a big part of the problem here is that there's no real right of way. The person who wants to opt-out of specific content often still wants the scene to progress; they dont want to STOP roleplaying, but instead want the content to fit within their guidelines. However, for people wanting specific content, there's an unspoken line where that's reasonable, but they dont feel like they should be forced to alter their preferred content to the whims of other okayers.
In short, there's a reasonable factor that someone playing vampires wants sex, blood, and rock and roll isnt truly required to alter their content because some player doesn't want to do that, but doesn't want to be left out of roleplay.
Opt-in is a great idea, but it only works under the assumption that players will accept being left out of scenes that they dont opt-into. RP on a MU isnt an entitlement. You can make the argument that in a perfect world everyone is welcoming and wants everyone' input, but the reality is that some games/scenes just aren't what a particular player is looking for and it isnt everyone else's responsibility to change for them.
This is why I dont play on Shang and avoided some other games.