Two ends to this - What I won't do to another, and what I won't have done to me...

To another - When I was younger, I had a girlfriend who really wanted me to be rough with her.  Normally, I have no problem with this, but she wanted me to choke her.  Not hands on neck, so she could feel the pressure, but hard, to the point of really actually choking her.  I could never to it.  Maybe it's squeemish on my part, but I just had a problem with it. 

The other thing was that she wanted me to hit her.  Not a playful slap, or even a little rougher, but actually full kinetic energy slaps and punches.  I'm 6', 240lbs, and she was 5'5, maybe 130.  I couldn't do it.  Maybe it's a bit silly, but it just went against my basic nature.  I couldn't hit a girl.  

To me - I will NEVER allow myself to be truly restrained.  I've been tied up, but always so that I could get out.   Another g/f tried to cuff me, and I just couldn't do it.   I have a VERY real fear of it, and will NOT let myself be that helpless in somebody else's hands.  Call i an emotional hangover from seeing some bad shit done, but I will NOT let it happen.....