Well, let's call them guidelines. I think I started by telling FancyFace she might be on safe ground telling a guy she had started being intimate with, that she has fantasies about being tied up. She might not have been comfortable bringing that up the very first night in bed together.
And if she really wants to have that fantasy become real in some way, it's best to bring it up early by saying something like, "I read a story where the guy tied a girl up during sex, and it seemed kind of hot." Then she could wait and see what he said.
He might say, "Not sure I'd be into that" or "Yeah, that does sound hot." Anyway, the topic has been breached with the second attitude making it open for discussion.