I can understand why you'd want to do it, in fact, I would might even convince myself to do it if I were in your position. As far as how far this 18 year old is going to take this, depends on her. She may just want a fuck-and-go as well, or she may end up being a psycho bitch and calling your wife at work, and showing up at your doorstep at 3 in the morning. Its kind of like playing the lotto, good luck.