I don't necessarily think you are "wrong", per se ... but I am one of those that believes turnabout is fair play. If you had initially agreed to finding singles, then have not made any initiative, then there are reasons for your hesitation and you do need to discuss these with your hubby, like you say you are, so he doesn't feel as though his fantasies/needs are being brushed casually aside.
I am
not bi ... and I *LOVE* the MFM threesomes!!!! LOVE THEM!!!

I
am inherently a kind of selfish person, and I would be perfectly happy doing MFMs all the time! :rollseyes I KNOW how hard it is to find a couple that you both click with. I had all but given up on the success factor of that particular endeavor, and was just going to try to "build" my own couple (lol) - a single male and a single female who we both liked and could play with together. Problem - my husband and I do not have the same taste in women and he doesn't like the ones I pick for him. Another problem is that since I am not bi, and since he wants me to be, when I do a FMF I want it to be all about HIM ... he wants it to be all about ME (just like the MFM) ... I say, if it's going to be all about ME, let me have it MY way and give me the man!!!! lol
Anyway, I digress .... If you aren't sure if you are bi, but you are wanting to explore that side of your sexuality, I believe a single bi-CURIOUS female would be the best option at this point. If you are still very unsure if you even want to try bi activity, a single bi-CURIOUS female is STILL the best option ... because you can communicate with the girl and let her know that you want to take it slow ... if she is just curious, and if you get the right one, she will probably be very understanding and will be happy to do the FMF in a way that centers around your hubby to start out and you can each determine your comfort levels. And take it from me, it is a
lot harder to find a straight girl than a bi/bi-curious girl!
I think by enjoining only a third in your experimental stage, rather than a couple, there is one less person to worry about and the three of you can go at your own pace. Then you can fulfill your husband's fantasy AND determine your comfort level and go from there.