Sex in Dubai without marriage is illegal and punishable under UAE law, it doesn’t matter that you’re in a relationship with your partner in your home country. It is against the law to live-in-relationship with opposite sex or to live together in an apartment or to share the same hotel room. Live-in-relationship means a living arrangement in which an unmarried couple lives together that resembles a marriage. But, they’re not married. Sex between such couples could lead to criminal sanctions in the UAE.
All sex outside marriage is illegal, irrespective of any relationship you may have with your partner in the UK. If the UAE authorities become aware (or, if someone lodges a complaint) that you’re conducting a sexual relationship outside marriage, you run the risk of prosecution, imprisonment and/or a fine and deportation. The UAE is in many respects a tolerant society and private life is respected, although there have been some reports of individuals being punished for sexual activity outside marriage, including homosexual activity, particularly where there is any public element, or the behaviour has caused offence. This applies both to expatriate residents and to tourists. Definitely, also applies to citizens.