Perhaps the media has coloured her view on what BDSM is? We are bombarded with (mostly incorrect) stereotypes.

I agree with Numinosity.

Communicate clearly...but to do that, You would need to make sure that the words you use to explain things to her, carries much the same meaning to her.
Perhaps give her some sites where she can read... even ask questions. Make sure that the information you present to her is clear, concise and true...not stories or fantasies. There are excellent books out there...

IF she knows how important this is to you, perhaps she will be willing to at least learn and try to understand.

Not foolproof by a long shot... but this way at least there is a chance of open communication and that is a very good start...