because (im assuming here sorry) like many subs what turns you on isnt just about the actual sex act it's also because you are doing what He wants, and i think if you were pushing your limits way too far or found something really sick then you'd suddenly find that you couldnt or even wouldnt go ahead with it.
and if you've previously discussed limits nogoes etc then unless you're unlucky enough to be with someone who doesnt give a damn what you want or feel you're unlikely to ever truly find yourself in such an extreme position.