It's bad to date a bunch of people at once.
John is dating Susan but sees other attractive girls at a party he likes.
John wants more sexual excitement in his life.
John asks Susan back to his place where he tries new romantic stuff with her in bed they've never done before.
John asks three other girls at the party to date him, and they say yes.
No other girls are hurt at all and Susan loves John even more.
Eventually all the girls are jealous of each other, and they all dump him, including Susan.