I met Caro in an online swingers’ community. She described herself as a slim, forty-four-year-old married woman, interested along with her husband in sex with men, women and couples. She often showed as active and nearby - three miles away, to be specific, which I found tantalising. But it was her single profile picture that I found really alluring - not pornographic, not even nude, just a beach shot taken from behind and...