I really like that post about boundaries vs rules. What it made me think of is accountability.
In other words, it forces them to shift their focus from "I'm mad because my wife won't let me ____." to "I don't want my wife to be sad so I will choose not to do ____."
Another thing for you to ponder: do have even the slightest desire to ever have a threesome? His libido aside, you have to do what's right for you. It may be his fantasy and that's fine. But if you know right now that you will never want a threesome, it's worthwhile telling him sooner than later, rather than letting you both think it's something you even can "work towards." Personally, it's not something I could ever work towards any more than I can work towards being turned on by being whipped: I'm just not wired that way, and all the talking and thinking and fantasizing in the world will never make me wired that way.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker