When she tells you that she wants to have a 3some with you, and speak in graphic detail on what she wants to do.. And then she talks about handcuffs and plan on where and when to have it.. and then when you're having sex, she takes you in the room and do her doggie style in front of a huge mirror..
And then she gets a conscience attack and says that we shouldn't be doing this.. that should get your mind pretty damn fucked up and wake up every morning with a woodie.
The mind is a powerful tool... say the right words and leave them to imagine. You will never get the fantasy out of your head.