Not having a sexual fantasy "with" the therapist. The therapist is not involved as a person. The idea of the female, any female as an all loving, always available, knows all one's desires, etc. (Think about fantasies of living with your therapist if you can; have you had those? You know how not related to reality those are? But they're there, anyway. Not wanting them or liking them doesn't mean you don't have them and they're good because they put you in touch with basic, everyone-has-them desires to be cared for utterly).

Sexual feelings are not a separate part of us, are not wrong, bad, or a "choice" they just "are". But they're not related to any particular relationship unless there's a sexual relationship there (like in the marriage)! So, talking about feelings, any kind of feelings, including sexual ones, is not about a relationship, it's about feelings and my feelings are different from your feelings and my feelings are mine. Just as you don't want your husband to feel feelings for his therapist (which are your feelings! and I'm not negating them or saying they are wrong or bad, etc.) you husband may feel feelings for his therapist. But they're not for the actual woman and a relationship, they are about his feelings and who he is as a man. His understanding those feelings and who is he as a man, could help him in his marriage.