Your girl loses a bet. She has to do one thing that my guy says without questioning it. He's a bit odd and tells her to bend over. Before she knows it there's something going on back there and she actually gets off on it. He doesn't even need her to be naked to get off and that's something that she can use to her advantage.
She could even be married and wanting to do more without even cheating so they do it more often and she gets off on it before long.
Willing to talk about other ways it can happen and I'm extremely open on who I PB and ages. Any takers?