No it’s not.This is one of those persistent difficulties with this nontraditional lifestyle.
Probably 50% of ‘open’ relationships will fail because of unrealistic expectations and a lack of communication, much like many people who try polyamory will end up disappointed.
I’m in an ENM relationship with my partner. We met through swinging so we both knew we liked a lot of sex. We talked a lot before we decided to become a couple and decided 100% honesty was needed.
She likes, and gets, a lot of cock. She’ll always tell me if she’s attracted to a guy (or girl) and it’s not unheard of for me to come home and find a guy balls-deep in her (I join in before you ask).
I don’t get as much variety but I have a girlfriend who stays with us most weekends who is also helping my partner explore her bi side.
If you find a thousand couples having an unconventional relationship they’ll handle it 1000 different ways but they’ll work because both parties are open and honest with each other and those they play with.