My mind works like this. Girl says I want to sleep in your bed but you have to promise me you won’t try anything. Man, at that point all I hear is “I want to sleep in your bed”.
You have two paths to take here. One, you don’t shake her hand and she doesn’t sleep in your bed. Two, you shake her hand, promise her, and she sleeps in your bed.
Option one gets you no action. And in option two, she might be very serious about the promise and you will get nothing either. But, option two definitely gets a girl into your bed, whereas option one does not. IME, you have a damn good chance that SHE will break the promise for you once she is in your bed.
Just take her to bed and be totally uninterested in her physically. But talk smooth and who knows, she might initiate something.
In other words, the odds are far better for you to get some action from her if she is in your bed than if she is not. Hell, even if you get no action at all, having a girl in bed with you is still lots of fun – way better than being alone. Ha, let her fall asleep and then *accidentally* pull the covers off her.
So yeah, damn straight I would have promised her and shook her hand. I think she wanted you anyway, from your story. I mean, you were getting kino from her, she was talking about how great it would be if her friend fucked your cousin (translation: if SHE could fuck YOU).
I would have been all over her like a cheap suit.