Why do swingers cheat on their partner

It makes no sense. ???????

We have been in the lifestyle 20+ yrs, and we have seen this a lot sadly, and this happened again at a party we were at last night.
Why on earth do people who have permission to play outside of their relationship, but within certain rules. Why do they ignore the rules, and cheat? I have never understood this, can anyone shead light on why people do this when all they had to do is follow agreed upon rules and they would be ok