I think any relationship into which one party enters with the presumption that something is wrong with them and the other party will fix it is adversarial.

I would be worried to hear about a therapeutic relationship that has no opposition. Opposition in the right dose causes a relationship to grow and a person to develop. (Think teenagers and their parents.) The presence of collaboration does not exclude the presence of opposition.

I think the OP's definition of opposition was not meant to be taken as the therapist is your enemy. (Some are, of course.)