you make it sound that you can be caught easily
My understanding is, in order to be convicted of having pre-martial sex/ adultery, a few things needs to happen:
you need to have 4 witnesses that witnessed the act.
and if these witnesses turn out to be lying, then they would be punished instead.
So actually, it's not so illogical or scary. Unless you are in a porno movie, or doing it in public, it would be hard to convict you.
when was the last time you let 4 people watch you have sex?