The idiom "in seventh heaven" means to be in a state of extreme happiness or bliss. It's an informal phrase used to describe a feeling of great joy and contentment

The phrases "cuckoo in the nest" and "cuckold" refer to situations involving infidelity and deception, particularly within a marital or romantic context. The "cuckoo in the nest" refers to an outside person, metaphorically a cuckoo's egg, intruding into a established relationship, potentially causing its breakdown. "Cuckold" specifically describes a man whose wife has been unfaithful. The progression from "cuckoo in the matrimonial nest" to a "ménage à trois" (a relationship involving three people) and finally to a "traditional (?) peaceful (?) Western nest" suggests a shift from a single instance of infidelity to a potentially more complex, possibly more accepted, dynamic, or even a return to a more traditional, yet still perhaps flawed, structure. 

"Cuckold in the traditional (? peaceful ?) Western nest": 

This phrase explores the potential for the cuckold to return to a more conventional, or "traditional," relationship structure. However, it adds a layer of doubt ("? peaceful ?"), suggesting that the traditional structure may not be truly peaceful or free from the shadows of past infidelity. The "cuckold" is still present, and the potential for past betrayals to resurface, or for the relationship to be based on incomplete information or assumptions, remains. 

In essence, the progression outlines a journey from a clear-cut situation of infidelity and its consequences to a more complex and potentially ambiguous state. 

It highlights the potential for infidelity to disrupt established relationships and how the repercussions of such actions can reverberate through different relationship structures.