I agree with Rhap, the errection might not be under his volunary control, however discontinuing the contact and removing himself from the source of his stimulation IS under his control. As the adult in this situation he should be the one setting safe boundaries. Smacking his daughter on the butt and letter her pull his leg hair while he is feeling stimulated, sounds like he may be a bit unclear where the boundary should be. I think it is good that you confronted him on this.