Sexual exclusivity is not a prerequisite for all relationships. ENM is a relationship based on the concept of sexual and emotional non-exclusivity. That's the entire basis of ENM. If you disagree, please explain what keeps these men in the relationship that they can't leave.
Why do you blame women for these relationships? Men and non-binary people are also ENM, but you seem to think it's exclusively women.
You know you're wrong because you haven't bothered asking why those men don't leave the relationship if they think it's cheating? If they were cheating, then you would be telling them to leave. But you don't. Why not? Would you tell a woman to leave a relationship if she were being cheated on?
Seriously, why is it exclusively the woman's fault and not the man's? Is the woman holding something in the relationship hostage? Children? Money?
That doesn't answer the question.