I have spent working in Emirates 6 years of my life. I have heard of people being caught living together and deported, but these cases are somewhat rare.
For example, you are a woman, and a local man wants to date you. He finds out you are in a relationship. He asks you to have sex with him and you refuse. He threatens to report you to police for having a relationship ( co-habitation) and if you again, refuse, he proceeds with the report and you and your boyfriend go to jail for up to 1 year, and both lose jobs. This scenario happened to about 5% of my friends, not more. Decide if the level of risk is appropriate for you, and good luck.
I had sex with at least 50 opposite sex partners during my 6 years in Emirates and have never had any trouble.