David returned his dumbbell discs and bars to the rack and strode out the weight-room doorway where Val and Leah were standing, smiling briefly<i>en passant</i>.  Finding the butterfly area unoccupied, he set the weights on one of the machines at ninety percent of capacity and adjusted the stool for height.  He sat down, pulling the crotch of his shorts away from his balls, and placed his hands behind the butterfly pads.  He began to use the equipment, feeling the beginnings of a burn in his pectorals.  For some reason he really enjoyed the sensation of power he got from this machine, and slowly began to increase his rhythm, matching his breathing to the motions of his arms and shoulders.  He noticed the two women claim a couple of machines on the other side of the room and appreciatively examined their bodies: one voluptuous, the other lithe and trim; quite a combination!  One bent over to adjust her seat and he began to feel a stirring in his loins.  He felt a twinge of self-consciousness as he became erect but, he thought, both of them had surely seen a man with a hard-on before....