I've been secretly going to massage parlors for happy endings. My girlfriend has no idea.
That sounds like trouble. Do you think its okay to cheat on your girlfriend at a massage parlor?
They just make me happy and I'm glad they're in this city.
It's terrible to engage in sex acts with other girls if you're already in a relationship. Your girlfriend would be terribly upset if she were to find out.
It's unfair to label my actions as a form of cheating.
If you're performing sexual acts with another person, then its the very definition of cheating. The fact that you're hiding this information from your girlfriend is likely proof that you know it to be true.  You should just be honest or break up with her before she finds out and ends the relationship.