I see where your confusion is. Let me explain it to you in plain words, if your relationship isn't exclusive (monogamous) then it is open (non-monogamous). Every other concept of a relationship falls under these two categories no matter how the partners feel.

Feelings or emotions have absolutely nothing to do with a relationship concept. The only thing that is considered is if the partners are married or single in whatever relationship concept they chose.

If your relationship is closed (monogamous) having an affair outside is cheating. But in open (non-monogamous) relationships, having an affair outside isn't cheating.

So when you say a monogamous marriage should introduce other loving relationships to spice things up, you are practically suggesting opening the marriage because the couple consent to it. But if you meant having a secret affair then that's cheating.