It's rather a wrong use of the words they feature.

Presented questions like "During the time you and [partner’s initials] have/had a sexual relationship, have/did you ever have any other sexual partners?” are not about that, and are indeed heavily limited to monogamy perspective.

You can be in a polygamous relationship, but, unless you previously agreed to have free sexual contacts outside your group, doing so with someone else without everyone's notice and agreement is still absolutely cheating.

Having sex with several partners in a predetermined arrangement is obviously not cheating.