It's bad to cheat on someone you're dating.
Sam's ex-girlfriend comes to town and asks him to join her for drinks, but Sam is currently dating someone else.
Sam wants to respond to her request.
Sam turns down the offer, mentioning that his girlfriend would disapprove if he did.
Sam gets drinks with his ex-girlfriend and then has sex with her.
Sam and his girlfriend maintain a strong relationship and get married.
Sam's girlfriend finds out and dumps him.