I am not certain which part you are upset about - that your therapist does not share your disapproval of an open relationship or that you have a different opinion from your therapist.

For me, I would think that if a couple is happy with the choices they make ("we have decided on an open marriage and it makes us both happy") then how is that not a perfect couple? (I actually do not believe in the idea of a perfect couple - but going along with the concept). Is it not possible to have perfect coupledom be that couple deciding what works for them and then having the ability to put that idea into play regardless of whether it fits other's ideas of how a couple should be?

But the other part of this is whether by not having the therapist share your view - is it that you are feeling disappointed that the therapist has a different moral code than you do and what would that mean in the broader sense?