Chess has rules, so you can cheat at chess.
Private eye explains how to spot a cheating partner | atcel.info
Cheaters in many cases has rules, so you can cheat on your spouse. What rule-based system are you talking about in this case? If you mean having sex with a prostitute while you are married to another person, almost all cultures see that as cheating. Many see it as a worse form of cheating than just sleeping with your neighbor or anyone like that.
And the rules of your culture are what matter in cases like these.
The only time they don't is if you and your spouse have made specific rules for your own marriage that are different from your culture's rules. So, if you have your spouse have freely made an agreement that it's OK for where to sleep pinay celebrity scandal blogspot a prostitute, then you can do that and it doesn't count as cheating.
People having affairs carry on having sex with their regular partners
But other people sex don't know about that agreement will still believe you're cheating, assuming you live in a culture that values monogamy as most do. Some people don't believe that private agreements trump cultural rules, have they might consider it cheating even cheaters the do know about your agreement. So, in a nutshell: If you haven't made a specific agreement with your spouse that it's OK, and you don't live in a where that says it's OK, then having sex with a prostitute is cheating if you're married to someone besides that prostitute.
And if you're not asking about cheating on a marriage, what kind of cheating are you sex about?