Why does our legal system condone sex for money if it is part of the porn industry, but regard it as criminal outside of porn?

It's a matter of the intent of the people spending and accepting the money.  When money is exchanged for sexual gratification, the sex is prostitution.  When the money is exchanged for a performance, it is not.  Police, prosecutors, judges and juries all know the difference, so don't plan on bringing a camcorder to your next session in order to beat the charges.

You went with your camcorder, documentation that illustrated that you were a photographer/videographer, (business cards, model releases, and a script)?
Remember, the burden of proof is on the accuser.

If you cannot dazzle them with brilliance,
baffle them with BS.

   Two things are infinite:
The universe and human stupidity,
and I'm not sure about the former.
   ~Albert Einstein