Ah, fairness again .
I'd think that the first step is to get him to clearly enunciate what it is about the MFM scenario that he doesn’t like. Without having a good understanding of his resistance it's hard to work on it. You may want to leave out the issue of "fair", because that can make it seem as though you are only doing the FMF threesomes for his benefit, which I am assuming is not the case. After all, if it's something you both enjoy, isn’t it automatically fair?
Now I know things aren’t always that simple, it may be that you really enjoy the FMF's, but for him it's the realization of a lifelong fantasy, which makes them somwhat more satisfying to him than to you (not to indicate that they are unsatisfying to you, just not AS satisfying), but playing the fair card can make t seem that way, and it might be part of his mental defense.
So rather than go for fair, just explain to him what you want and why you want it. Maybe share with him a fantasy involving two guys while you two are in bed together. I can't speak for your boyfriend, but I suspect most guys have a hard time not getting turned on by a scenario that their girlfriend is getting obviously worked up over. Once it's entered into his sexual paradigm through your fantasy it may be a shorter trip to making it reality. Porn is another good avenue for that to take place. Watch a porn with him, and when the inevitable MFM scene comes on, let him see how hot it makes you.
Personally I can't picture the scenario which would whip my wife into a masturbational frenzy that I would then say no too. Remember, we (guys) are conditioned damn near from birth to judge our sexual prowess by how well we satisfy women. If he sees how much the thought turns you on, there’s a fair chance he's going to want to be the one to give it to you.
Edit - Oh yeah, and "Hi! Welcome to the board!"