Being real a minute though, it's not uncommon for girls to back out of FWB situations because they're getting too attached and don't want to be clingy and ruin a good thing. She may have the same issues you do for the same reasons: fear of rejection and ruining your friendship.
Us guys are plenty fucked up in the head but there's nothing saying we have the market cornered on insecurities and fear of rejection. Play it cool, be relaxed, let her know you don't wanna mess up the friendship but you just wanted to run something by her. Tell her in a relaxed but honest way, and most girls won't wig out, and are actually pretty understanding.
