It's wrong to sleep with two people at once.
John is a young virile 23 year old with a serious girlfriend and an insatiable sex drive.
John wants to release his sexual energy.
John has a long and intense session of romantic lovemaking with his girlfriend.
John begins sleeping with a girl he met at work in addition to his current girlfriend.
John feels much better having released all of his sexual tension and his girlfriend is impressed by his stamina.
John's girlfriend discovers his infidelity and leaves him immediately.