i find such rule to be inherently controlling and coercive. and used to punish abused and neglected partners who find affection elsewhere but can't leave their relationship due to coersion or fear of homelessness. Cheating like that should be like "we agreed were both in a diet, but I had a donut that someone brought to the office", not "you are inherently a horrible human being who deserves to be shamed and hated for seeking human connection".