For the record, four people heard our conversation, guitarist, drummer, singer, and me. We were in a huddle with the mics muted while the guitarist switched guitars. I'm not an idiot. I am a BL in another project and I know how to "say it loud with a whisper".
That's cool -- sounds like you did it fine. I was referring more to other experiences related in this thread of not playing, or playing half-assed, or playing under protest and communicating that protest somehow. All those things are visible to the audience and diminish the band's performance.
There's a clear line between don't like and HATE. And if the song that is hated doesn't add anything to an already energy filled set that does a nice job of sparking the audience, why bother?
I guess the "why bother" thing to me goes back to why you're on stage in a cover band in the first place. Is it for you, for the band, for the club owner or promoter, or for the audience? Ideally it's a confluence of all those, and happily it usually is. But in my mind, if there's a conflict at any point, the audience should come first. (Then the band, then me, and lastly the owner/promoter!)
I don't think I could muster up true hatred for any music that is not racist/sexist/etc. like others have pointed out. That seems to be injecting too much ego/judgment into the equation. And it's still the same bottom line... even if the band starts playing something that makes you want to puke and even if you've talked about it ahead of time, just suck it up and go with the flow, play your ass off, and enjoy the appreciation from the audience. You never know who's out there who could lead you to the next great gig (maybe even the fool who was requesting the stupid song!).