Not all relationships in 2015 look like the Leave it to Beaver 1950's nuclear family version of relationships. Unfortunately as the mental health fields began to splinter and then establish themselves in the 1950's, we are stuck with licenses like "marriage and famly therapists". The agenda is just built right in there isn't it?!
There is nothing wrong with monogamy and nuclear families and the therapists that work with those sorts of relationship structures. But it is important to note that there are whole segments of the population that don't have these sorts of relationships, and these folks have the same relationship conflicts as everyone else.
In an open relationship? Monogamish? A throuple? Poly? Some therapists are prepared to work with you... but some aren't. This is why the intake session is so important in starting your work with a therapist. This is the time where you get all the factual stuff out of the way- like why you're coming to therapy, what your family structure looks like, and how you'd like to see it improve. If you feel like the therapist gets it and your work together will be effective - then awesome! But if you feel like the therapist winces every time you refer to your pansexual, poly life partners or your adopted child that you'd like to raise vegan and gender neutral- than don't force it! There ARE therapists out there who can help you learn some evidence-based strategies to help decrease your conflict and improve overall happiness and well-being in whatever relationship type you have without forcing their morals on you.
So don't let the terminology get you down! Couples therapy for triads. Relationship therapy or family therapy for non-legally married life partners! It's 2015, folks!