It’s very true that monogamy remains one of the last unquestioned bastions of relational legitimacy, or at least in the minds of many couples’ or mainstream marriage therapists.
This is not really helped by the natural pro-monogamy bias found with many therapists and is not an accident. In the majority of conventional counseling programs, therapists-in-training are taught that monogamy is important and should be protected and so the problems begin.
Conversely, non-monogamy is cast as a sign of a problem within a relationship and flagged as something that should be solved instead of investigated, celebrated or explored.
The Active vs. Passive Bias
Some therapists either don’t know about or can’t get their head’s around consensual non-monogamy, and thus exert a passive bias based upon a lack of knowledge, experience, and understanding. Other therapists are actively hostile to any form of non-monogamy—consensual or not—and treat it as the ultimate breach the couple must resolve before anything else can happen.
The following links are probably quite helpful for those in search of some poly enlightenment.