Anyway, Yamada is still wrapping her head around the concept of lust, that she feels but doesn't understand as normal. Kase on the other hand is affraid to scare her. Maybe happened with an ex.
Possibly. All former relationships she's had are with strong, tall, athletic runners. Girls who were all on the same wavelength as her and could meet her head on. Here comes small, shy, rather non-athletic Yamada, and Kase-san just don't know what to do with herself. She's not getting the responses she's used to (for better or worse) and it all feels almost like a first love all over.
It was hinted, when Kase stripped in front of Yamada, that her previous girlfriends, while ok with the concept of "going out with", or dating, a girl, weren't ok with seeing her nude and that it went downhill when she "tested" them that way. At first, she thought that Yamada was reacting the same way, but it was only Yamada being Yamada and overly shy.
So, while Kase is not alien to the concept of lust and knows what she wants, I somehow doubt she ever went very far with her previous girlfriends. Her previous relationships probably fizzled at that point and that's why she's utterly scared Yamada is going to run away, because, well, she really loves her.