It is rude to date someone while dating another person.
Dave who has a girlfriend is out with friends at a mall when he's asked on a date for that night by another woman.
Dave wants to answer the woman's proposal.
Dave tells the woman that he's taken, and he cannot go out with her even if she's pretty.
Dave tells the woman that he'd be glad to take her out that night and gives her his phone number.
Dave's friends tell Dave's girlfriend who is happy to find out Dave is capable of being faithful.
Dave's friends tell Dave's girlfriend who is shocked that he hurt her so much and cheated on her.