My experience has been that regardless of how the relationship starts, it will evolve on its own.
CG and I started as FWB and seem to have remained at that stage, although I briefly started having more feelings for him which then fizzled out. His wife would like for us to be more closely connected, but it's not something we can coolly decide.
TKO started decidedly as FWB, actually as play-partners, and has evolved into a fully poly relationship including a great connection with his other girlfriend. Who would have thought?
I guess when you meet new people you can't say up front what type of relationship is going to happen. Unless it's "love at first sight" (in which btw I don't believe), any relationship will tend to start as friends, evolve into including sex and/or and evolve into including feelings maybe in reverse order.