The previous drama sounds awful, and I totally understand why you feel uncomfortable with the prospect of them dating again.
Originally Posted by CageyCate
I try to play this out in my head. What if I have no contact with her? If we are just honest and say we don't like each other and we won't interact? Would that work?
This would be the first thing for me. I would honestly express how I feel about her, and about them dating. I would want him to know, and also to hear what he has to say. If he wanted to do it, I would not try to stop him (because I just don't see it as my place). However, it would cause two things.
Firstly, I would make so strong personal boundaries around it. That is, exactly the kind of things you describe. E.g. I don't like her, will not pretend to like her, and will not be sucked into her drama and, thus, will not socialise with her. There is no requirement to be friends with metamours, or to like everybody your partner likes. If he tries to pressure you into having contact with her, I would take that as a huge red flag of him not respecting your feelings&boundaries.
Secondly, it would make me evaluate my relationship with him. There is no getting around that, difficult questions would arise. He knows what she is like. He knows she would rather have him for herself, he knows she does not respect you or your relationship with him, he knows dating her will likely lead to drama. Yet, he wants to date her. What does that tell about him? Is it lack of consideration? Lack of respect? Prioritising his feelings of love/lust/addiction over stability of his current relationships? I would think about these things in the context of his previous actions (such as having the affair), and consider what that tells me about my partner and his trustworthiness. Thus, even bigger issue than the potential drama-ridden relationship would be my partner wanting to have such a relationship.