I had a read through your older thread; when you first came here for advice. I'm really glad that you and Mike are now above board.
I don't mean to sound judgmental... but I am being ~grins~ .... I do question Mike's ethics. Not everyone in polyamory is ethical and not every monogamous person wants to cheat. But, for me, poly or mono, ethics are important. The *only* time I *personally* would completely keep an entire relationship a secret from one of my partners is if they specifically expressed a disinterest in knowing. For example, a casual partner who really didn't care what else I got up to, outside of them. Withholding *details* of what goes on inside relationships, I really don't mind at all. And actually... I don't always think as secretive or unethical people as selfish or monstrous. Often, I find that those who tend to lie tend to be quite cowardly in nature. That can be a concern in the bigger picture. Food for thought.
Anyway! What you seem to be facing now is Sample B of your polyamorous/polysexual journey. For me, Sample A is issues surrounding jealousy. Sample B is issues surrounding control.
We've all been there, I'm sure. We've all disliked a metamour. We've all perhaps even been affected badly by a metamour, in terms of the domino effect of their actions. We've possibly all done *something*, large or small, to try to take control of that situation. It could be giving advice that wards our partner away from this 'toxic influence'.... right down to the horrid, horrid Veto rule: "you cannot see this person any more" or (even more of a killer) "choose between us".
It's normal to want to have control - particularly when we are in emotionally stressful situations. It's what we do. Actually, life can feel a hell of a lot better when we relinquish control. (yes, yes, completely hypocritical, since I'm a Domme... but, hey, I think bedroom control is the good kind... ~grins~).
Issue 1 -
Cassie doesn't know about you and Mike.
Reason? Because "she's too whiny and jealous".
Real reason - because Mike is a bit of a chicken.
Polyamory/sexuality isn't only about getting your rocks/lovehearts off with multiple people. It's not just about 'not feeling jealous' (I feel jealous all the time). There is a whole other side, that I actually find equally, if not more, important. That side is COMPASSION. Nurturing, understanding, taking responsibility, balancing your life, being fair to other people, resisting the urge to only think about yourself.
I don't know Mike - I'm not saying he isn't wonderful and sweet and fun. He could be the most lovely, caring guy in the world. (In fact, he most like is sweet and quite passive; judging by his conflict-avoidance; the way he puts up with a seemingly high-maintenance secondary and the way he avoided flirting with you when you were monogamous).
Issue 2 -
You and Peg think Mike should dump Cassie.
Reason? She's toxic.
Real reason - you and Peg cannot let go of the urge to want to make Mike's decisions for him.
I don't mean that rudely. I have many moments where I just want to throttle some of my GF's other partners. I have moments where I wonder what on earth she is thinking; why she is willing to put up with so much crap, etc. She has many, many moments like this in terms of my choices, too. When we care for someone, we don't want to see them unhappy. We want to protect them.
But... consider this.
Mike is getting something from Cassie. There's something he thinks is worth the trouble. It could be sex, it could be emotion, it could be the thrill of drama... it could be anything. I think it's to do with feeling needed.
It's wonderful that Peg feels comfortable enough to confide in you. This means that you can both vent about your feelings, without burdening Mike with them. Hopefully, Peg has said all of this to Mike himself - since he is the one that needs to hear it.
But if Cassie has affected their marriage in some way, how is this all Cassie's fault? Even if she's a complete shit, Mike is the one choosing to date her. Peg is the one choosing to stay in the marriage, despite feeling that Mike 'shouldn't' be dating her. Both of them decided to let her move in - and to support her whilst she wasn't working. Mike and Peg are the two people responsible for what is happening in their own marriage - not Cassie. Both of them could take steps to drastically cut down the impact that his secondary relationship has on them: one of the most helpful steps being to create an environment of distance and autonomy when it comes to that relationship. No whining about Cassie drama, no inviting Cassie to the house, no avoiding responsibilities because of Cassie, etc.
There's something about drama, you know. It is like a drug. I've been involved with some dramatic women in the past and they have a way of hooking you in. I have this need to help people. I'm a fixer. I used to work in the Psychology field and this is simply in my nature. Curse it! Because it attracts selfish or needy people to me like moths to a flame. And dramatic people do make you feel needed. If Cassie is somewhat emotionally unstable (and it sound likely, after suffering the abuse from her husband), Mike probably feels like she is a victim and that he needs to help her. He may even enjoy the attention, the intensity, or any other by-product of it. There is also the endorphin rush of drama. Why do you think make-up sex is such widely recognised treat? When we fight with people we care about, we are overcome with negative emotion. When we make up, we are flooded with joy, relief, happiness, excitement. It can be very difficult to let go of the drug-effect that dramatic people bring into our lives.
It could even be that Mike *has* lost interest in her as a lover but struggles with assertiveness and really, really, really wants to help her.
I know that it is not for me to tell him what he should do in his relationship with Cassie. It's totally his decision. I get that.
~grins~ mmm hmmm... you're right...
A few final thoughts on what you can do about this...
As GG said - honestly, if it brings something negative to you, ask him not to talk about it with you. Even better than that would be to focus on the *good* feelings that being there for Mike creates in you. Same for Peg.
Furthermore, help Peg by ensuring that she is making her voice heard to Mike. If Peg feels that Mike's relationship is damaging their marriage, she needs to be fair to him and put this out there, very very clearly. They absolutely do need to go to therapy and find a way to deal with this.
Overall, this is a topic I've been thinking about a lot lately and I do completely understand that it's a tricky one. We are all affected in some way by our metamours - in minor or major ways. If we don't believe in controlling our partners, we must either be there to offer counsel and support, or completely remove that option and insist that our partner deal with this problem on their own. What is unfair to *everybody* is allowing a partner to keep using up our support - but mentally punishing them for carrying on with the relationship.