Something I have noticed over the years, I do interior design, is that people make a lot of declarative statements that really don't convey what they really mean. For example I don't like brown, I would investigate what it is your fiancee doesn't like about brown does he think it can't be used to look nice, does he have bad associations with brown? It can be that without visual cues some people cannot imagine the possiblities.
I would use some of the inspriation boards as well as the real weddings on this site that use your proposed colors (and/or make a board yourself including a couple pretty brown gowns) and ask him his thoughts, my guess is that once he can see more complete visuals he will come around. If he really hates brown and even when seeing how nicely it can be used as a neutral I would rethink it all together.
Finally if you get past all the brown phobia brown dresses and black suits/tuxes will look nice together. You could also have the dresses be in blue or sage if you found you don't like the brown/black combination. Again I would look for pictures so you can also decide what you think.