Unknown Rule

Not sure if this rule exist anymore but ladies when is it ok to date someone who has already dated your friend. I always known this to be an unspoken rule that my friends exes are off limits. How could you talk to someone who your friend was just all giddy about,who your friend could not get enough of and most likely had sex with. But yet you turn around and start talking to the same guy, I'm confused. I understand there are more females then males but there are enough males out here that you do not have to have your friends leftovers. I'm really trying to figure out how does this go because majority of the time females tell their friends about their guy and their emotions they have for him. Also we all discuss the intercourse we have with our mate there is no doubt about that. So did you become curious about what your friend told you about this guy that you decided to see for yourself? Or was he that cool of a guy that you thought maybe if he was yours your two relationship will go further? Or maybe you're that disloyal that no one is off limits. When being someone's friend there is a thing called loyalty and dating your friends ex is disloyal. No matter if she told you go head but for the simple reason that you thought it was ok to ask or even pursue it shows that you cannot be trust especially after your friend has went into details with you about this man. I'm seeing this more and more with females like why would you want to have what your friend had. Sharing is caring but not in this sense. This act alone shows your friend what type of person you are and how you cannot be trusted or maybe I'm seeing it wrong. Can a friend talk to their friend's ex and it be ok?