It's as old as the hills... definitely as old as therapy itself. It's often a very healthy progression in therapy.

I've had fatherly sex fantasies for as long as I can remember, so that **** doesn't phase me. Most of how we relate to others and the relationship patterns we form, stem from our relationship with our parents. So it's a good idea to look there first.

I think it's a complicated mix of many things - could be symbolic of your desire to work together. Could be he's hot. Could be nice to have a man actually listen for longer than 5 minutes, without interrupting.

You should talk to him about it though because there's no point in hiding stuff in your therapy... it's for you after all. And it won't magically go away. Most therapists will be fine with it and respond in a kind empathetic way. Unfortunately not all are great with the working through bit.