Yeah, welcome to swinging. That's one of the big silly double standards. Everybody want a bi girl but bi men are generally not welcome. Plenty of people won't go anywhere near a couple if the guy even admits to being curious. "Bi guy" is high on the list of negatives that your local community will pass around when they talk about the two of you if you're open about it.
It would be nice to think that there are areas that are backward about this and other areas that are more progressive, but I live in South Beach. If it's like that here then I kind of think it's like that everywhere.
Two bi couples together can be mind-blowing but it's really hard to find the right couple. If you guy really is curious then I suggest looking for a single bi guy for an MMF. Plenty of very nice and very hot single bi guys are very willing to play by whatever rules you lay down just to have an interesting night with a couple.