honestly, it is illegal and you could be arrested, but most likely you would just be asked to leave the store. I doubt if the employees really care enough to get law enforcement involved. At most they would notify the local security, and they might notify the police, but they would probably have to verify the crime first.

And technically, it's your word against there's. Yes, you were in the dressing room with her. But how do they prove you were having sex? You could have just went in, to tell her how she looks in her outfits. If you never admit you had sex or did anything that could be construed as a crime, they can never prove it, and they can't arrest you without evidence. They will just ask you to leave, maybe ban you from the store at most.

It's a very risky activity, but people over-estimate the power of employees or local security. Employees usually cannot do anything beyond calling security or asking you to leave. And technically a security guard can only arrest/hold you if he himself has personally witnessed a crime. Most of the time they simply escort people out of the mall, short of verified theft or violent acts.

Keep in mind I am speaking only on my knowledge of California law and I am no expert, so don't take my opinion as fact, and laws will vary depending on where you live. Also I do not recommend engaging in such illegal activities, I am simply commenting on the subject not endorsing it.