But by the sixth chapter the situation is different, Bella is already aware that it's no longer just a sexual need and in fact wants to deepen the relationship with MC.
Bella knows how important Mc is for Jill, so important that she has to submit to Tybalt's blackmail, to accept not to see Mc in public. how can a friend bring him home and ask him for a real date?
in this situation how is the escapade with Cathe an insurmountable problem?
My take on it is : Bella is struggling with her morals and her desires, maybe even her trauma's....
She has a hard time stopping MC when he kisses her after that "missed since drunk as a skunk dinner date"...
After you go "see Bella" in the library it is her saying she's married but it's her that keeps pulling MC closer...
After she helps clean MC's room in the DIK mansion she is in thought "you've become what you hate most... An adulteress"..
In a way MC getting into a relation with Jill is hope... Hope, since she herself can't seem to make the choice she thinks she need to make... She allready acknowledges her desires are stronger then herself...
Jill is very much opposit to her in that regards.... while Bella gets pulled into the relation with MC due to her feelings but despite her morals, Jill's Morals prevent her feelings to make that step, every little thing is a possible obstacle to just not call MC, or send him away, or whatever else just not to tell her feelings to MC and getting to the physical stage of the relation...
Bella's help to bring Jill and MC together might mean her way out.....
In that light adding another relation between MC and Cathy, this would even mean her way out is in danger.... accepting there can be more then 1 physical relation at the same time would mean there is no hope for Bella...