Psychotherapy, as I see it, should be about helping people understand themselves better or raising their self-awareness. I don't see what your therapist did as therapeutic and consistent with the purpose of therapy. She would have fulfilled her role much better by trying to understand your marital problems and what kind of emotional needs your extramarital relationship is fulfilling that aren't being fulfilled in your marriage. People don't have affairs for no reason. No statistically average person intentionally wants to hurt their partner, and affairs usually don't happen because the cheating partner lacks sensitivity or consciousness. Usually, cheating partners experience a great deal of guilt, but cannot stop because they don't want to go back to the miserable life they had before the affair started - exactly what you are experiencing. This is not to say that you shouldn't own your choices and your behavior, but to acknowledge that life is much more complicated than a simple "right" and "wrong" polarization, especially marital life and everything related to human relationships. Your therapist is in a perfect position to have this basic life wisdom and to stay above primitive judgments. In fact, she is in a position to have much more wisdom than a regular person who is entrenched in their conventional judgments.

All in all, no, she should not impose her moral standards on you, but should try to understand you instead.