Didi will be monogamous. Once she's latched onto her lover, she has proven to be quite faithful time and time again. It is only when someone new or better comes along
and the 'previous model' doesn't provide her with an orgasm/satisfaction does she drop him/her like a hot potato. She might dump them rudely or unceremoniously, even forget their name, but she does at least make sure there is a break made. I'm pretty sure most of us readers would be interested in her take on multiple partners in a commitment-free environment, one of whom is her lover-du-jour, but that is for a future strip I reckon. I'll give Sonya a good mark for effort, though.
Zii on the other hand, I feel, will be the one to screw up her relationship with Didi. Whether it is the obvious reason of Zii not being able to keep her hands off of those she finds attractive/sexy or because of her deep-seated discomfort of commitment, she is the weakest link. In this case, Didi's emotions & morality are strengthening her resolve. Didi let down a friend/lover and she doesn't want to repeat. Zii is just a horndog who is happy with her favorite toy; dangle the right temptation in front of her and she will give in to her lusts. It is conceivable that she learned a little bit from the sadness brought on from losing Eric; I expect she will learn more from rebuffing Sonya.
As far as Sonya's next plan of attack, I guess that might depend on if she saw Zii diddling herself while seducing Didi (or heard Zii asking if she could join in). Sonya knows Zii's personality well, and she doesn't have much to lose now. While Sonya likes to play bass apparently, she was in the band to be closer to Zii. If that pains her, she'll drop it or risk it all to get Zii back. Or I could also see her transform into a very mean and spiteful bitch. Either way, I expect anger to be her next reaction and to kick Zii out. She'll need some time to stew on the events, become more bitter, and re-strategize. One wonders if circumstances will present themselves for Annie Conda (or her 'evil' boss) to partner up with Sonya in this endeavor. /cue
Imperial March.