Thanks, Black Unicorn.
I don't personally much care for the latter definition, because I think there are plenty of monogamous people in love with multiple people who yet choose not to go poly because they believe it's wrong, impractical, too challenging, too threatening or simply not done. Which is btw why I don't think legal polygamy would have an effect on cheating.
I know what you're saying...that even if it was legal, people would still disagree on moral principle, or because of the financial burden. So they would still rather be cake-eaters. And even some of those who would choose to have multiple spouses, would still be enticed to go outside of their marriage(s) to that which is forbidden.
Like David and Bathsheba.