Is there no scope for compromise in this situation?
I realise that there shouldn't be the requirement for such a conversation in a band, but this issue isn't of your making, it's been forced upon you, so the best situation can surely to be attempt a compromise.
If you don't feel comfortable in relinquishing full bass duties (and why should you, really?), and you are not wanting to take over drums or guitar or whatever other instruments are already played in the band, suggest a new one?
Tell them you'll play gong throughout the song? Or triangle?
Try and make the suggestion just ridiculous enough to highlight that their suggestion to you is equally daft and offensive.
You may not be one for confrontation, and that's fine, I can understand that if that's the case, but again, this is not a situation of your making, they brought this to you, not the other way around.
It doesn't seem as though you are quite prepared to walk away from the band, the posting of this thread says as much to me, but sadly you'll have to deal with the situation one way or another.
Mark