I dont get this type of saying in regards to sex?

The ole saying of "if u had sex with person A, then u've had sex with the people person A was with" I am not really clear what that means? How can u have sex with someone if u never did?? Like i just had sex with a girl name "Tiffany" And shes been with 10 guys.. Then that means i had sex with those 10 guys.. Im really confused???