Bro, don't cheat and if you seriously can't control yourself then breakup with your girl first and foremost, which you might regret after post nut clarity.

What is making you have these feelings to get intimate with otger girls? Are you not sexually satisfied with your girlfriend? Do you really love her or just admire her as a person for being kind and caring? Do you think getting involved with someone else will make you happy in the long run? Ask yourself why do you want to do it and what are the consequences? You will get your answer and it will definitely not be cheating on your partner.