Of course Cuckolding can ruin a marriage/ relationship if it's not open. What began as fulfilling a fantasy can quickly become an issue of control and resentment. For most, it remains an occasional fantasy or in reality, a sexual activity they indulge from time to time. Men and women can get incredibly self-absorbed and demanding over this fantasy into reality, ultimately leading to the demise of their marriage/relationship.
In the beginning, cuckolding may be cool but one day you may wake up full of resentment, anger, and sadness, especially if you were not honest that part of you didn't like what you were doing and he wasn't honest when he said he was okay if you wanted to stop.