Late on Saturday night, I suddenly get a call from Yu, my girlfriend’s adult sister: she has missed the last train and wants to stay over. I think I refuse, but minutes later the doorbell rings and she lets herself in. She has been drinking, stands even closer than usual, and keeps touching me while her loose neckline shows off her chest. She presses her breasts against me so deliberately that it feels like an invitation, even though discovery by my girlfriend would be disastrous. When I warn her that they are touching me, she admits with a teasing smile that she is doing it on purpose—and I realize my self-control may be finished.