When you are married and having an affair it’s considered as cheating. I for one too believe so but some debators have a different view. I once had a conversation with my ex girlfriend about similar topic. She then shared a story about few guys who argued about the same thing. They said if both of you are not married, having an affair is not cheating. It is just exploring. Their argument is there is no vow as far as marriage ni concerned. I would say simple because you both still tick single when filling out forms.
Maybe you will also object to say when two of you agree to give it a try it’s almost a vow once you’re in a relationship. That is if when you set out those prons and cons it becomes cheating, I think. If one of you break those promises. I really don’t know.
It all brings down to the purpose and meaning of a commitment. However, it doesn’t mean you’ll never get attracted to other people. I always say even if you’re considering polygamy disclose that at the beginning of the contract so that the other person knows what is she subscribing to.
Cheating is when one of you is married and decides to have a affair without consent of your partner. However, no one ever want to mention his/her loose pants from the beginning. It’s always out of the other person’s preferences to date other(s).
Adultery is often mentioned by Church people. In any way let’s also touch on it. It is when two people married to their spouses and decides to sleep with or having an affair with someone who’s also married. It doesn’t sound like adultery if only one is married and the other single. Hence I’ve never seen it in the Bible where it prohibits polygamy.
New Testament folks will quote, 'if one considers the office of a Bishop, he should be husband of one.’ they sometimes want to replace 'bishop' with 'anyone or everyone.’ But this applies to bishop candidates. Sounds funny. I know. What if none of us wants to be a Bishop? I don’t know.
The way I see it if no commitment or vow let’s not take each other to any court if found sleeping around. I am kidding it is all up to individuals what you want out of your relationship. It is also up to your moral values. But table out all of those at the beginning.