I have done this before. I wouldn't sincerely call it cheating. When a person dates someone. You ask that same person do you think that guy or girl is good looking. They kindly respond with "I have a boyfriend" that does not mean you are dead. People deny how attractive other people are while their dating someone then when they break up.
They start the cycle again with a new person. What if we were just honest. I'd said you could sleep with that guy or girl since it's just sex and not emotional as long as you came back to me every night. The person adds spice to their sex life given they are both trusting and secure. But wait some might say. "Isn't this cheating" I look at is if you were not dating your partner would most likely do if single.
Why not enhance their pleasure giving them the option to do it while dating you. It it enhances their pleasure and afterwards they come back to you. Then who exactly is losing out. People bitch and moan about their personal relationships sometimes because they become bored with same sexual positions and situations. This is just another way of enticing a love life.
Granted not for every relationship depends on personality etc. But a person who is less jealous and controlling in a relationship lasts longer then an abusive boyfriend/girlfriend creating stress mentally. Just my two cents.