Is there a reason that goo's can not go any further with Bianca other then the fact that they are not considered bipeds, even thought they now have the ability to become bipedal?
Her familiarity is capped at 50 in Part 1. Familiarity is uncapped to 100 in Part 2 because Part 2 contains an option called [Cuddle] which advances the relationship, and Bianca will not cuddle with Taurs or Goos, ergo no advancement. The stated reason for this is that she wants to be able to entangle with someone on her own level. She can't put her heart into it unless she's grasping against flesh and bone, knotting legs etc.
In other words, it's simply me being strict for her. Goos will be able to engage in regular sex in random encounters though, so they'll have something over taurs.