I didn't read the whole thread-
and I tend (at this time in life) to prefer the "full on, full time, live together, everything shared" relationships.
HOWEVER-that doesn't mean that other types of relationships are 'legitimate'.
My boyfriend and I were FWB for over 10 years!!! We had no intention during that time of taking it further. We loved each other as friends, we had great sex on rare occasion.
Circumstances unexpectedly changed and now we're living together with my husband and co-parenting the kids.
Don't sell yourself short with unnecessary limitations.
Just because SOMEONE thinks FWB is "casual" or "too casual" for them-doesn't mean it has to be for YOU.
Just be sure that instead of discussing labels with potentials-you clearly state what you want (and don't want) from a relationship with them RIGHT NOW (no one can predict the future-it could change tomorow, ten years from now or never).