Books, films, even soaps don't just reflect society, they also influence it. It is often insidious, a gradual shifting of attitudes.
Do we believe that the desire to inflict violence that would normally be considered torture is a positive attribute, to be encouraged and freely explored. Even amongst young, sexually naive people who often take their cues from popular culture?
Do we really believe such thoughts and behaviours can be strictly confined to the sexual arena, and strictly confined to relationships where there is truly consent given freely. I believe that spending time thinking about ever more inventive, and in some cases extreme, ways of inflicting pain is generally corrupting to character and behaviour. Thoughts are powerful.
I want to live in a world characterised by love, not one where people torture others in the name of lust. That isn't what I'd call a civilised society
The 'it's just fantasy' argument: How comfortable would people be if their friend confided that they indulged in sexual fantasies about children. If their friend said they would never carry out these fantasies, they know that wouldn't be right, they never look at sexual pictures of children online, it's all in their head. Would you be thinking, that's all right then. Or would you be deeply uncomfortable that they are stoking their tendencies, giving them 'headspace' and developing them, rather than trying to quell or suppress them?
This is just how I am: I'm sure there are those with innate BDSM tendencies, but people can choose to nurture them or control them. It's not just how you are, it's how you want to be. We don't all give in to all our urges regardless. The question is whether BDSM is really a good thing/harmless to its participants and society at large, in which case there's no problem with indulging the urges or whether we as a society decide, as we do on many other issues, that it impacts negatively on society and we will control it, or some forms of it.