There's no way that it could be logical or healthy to be okay with cheating but not with ethical non-monogamy.
However, the pseudo-logic behind it may come from the person's understanding of romantic love. If she believes that it is impossible for someone to love more than one person, than she would not be able to believe in polyamory. She could, however, believe that the married man no longer loves his wife (maybe he never loved her at all because she's a horrible bitch!) and only loves her. They love each other and are therefore MEANT to be together, even though he's still stuck with his horrible wife, the poor man!
Tied into these ideas is, I think, the societal notion of women-in-competition-for-men. If you believe that women are supposed to be competing with each other for men, then it's okay to steal a married man by cheating with him. But it's NOT okay to have non-monogamous relationships where women can be honest about being with the same man--and can in fact be friends with each other.
I have met SO many people who find the concept of polyamory and/or ethical non-monogamy absolutely horrifying because they cannot fathom how someone could be okay with "sharing" a partner.
These people are not okay with cheating either, but they would find it easy to grasp the concept of someone leaving their spouse for someone else. They would NOT find it easy to grasp the concept of someone being okay with their spouse having polyamorous relationships.
It's kind of nutty, isn't it?
For the Original Poster: if this issue has immediate personal significance for you, you might want to post your specific situation in the Poly Relationships section. You could get specific, helpful advice rather than a general discussion.
Single, straight, female, solo, non-monogamous.