I would say because it just is the way it is. It is all a part of the transgender feelings. Just like your eyes are the color that they are. (Sheila ... I am not saying that your SO experiences this.But the conclusion in this thread is that the majority do, to some degree).
But for CDs to say they are not attracted to men since it doesn't count when it is only present when they are dressed, and because of this there is no reason to involve the SO, does not help the SO deal with the issues that she faces.
And the other major point is that the degree of attraction would be individual. It is worth repeating that for some, it is an occasional, private, fun fantasy that doesn't take away from the sexual relationships with a wife. For others, it is a stronger fantasy that may cause issues in the bedroom with the wife and if this is the case, the wife needs to know this.