I think it's hard to hear the word "fantasy" as not harsh. It just has connotations of "all in your head" and divorced from reality. But I think all relationships have elements of fantasy, in the more neutral sense of seeing attributes in the other that we choose to see (projections of our wants/needs perhaps). As long as the attributes aren't too far off the mark or destructive, and the other accepts and adopts them, the relationship benefits.
Maybe this is what is going on when people say a couple "grows together." Each makes a choice to modify their self ( in ways that don't conflict with core values) to become more of what the other wants/needs and it can lend stability to the relationship.

In a therapy relationship, the same dynamic can happen--and is maybe even heightened for a therapeutic purpose. It's more often unconscious for the client (and occasionally so for the T), but if the situation allows, the dynamic can become conscious and a lot can be learned through that process. While the purpose may be therapeutic, I don't believe that means the relationship is fake; on the contrary, I don't think therapy can be successful unless there are genuine feelings on both sides--even if they're often not the same feelings.