Cheating is breaking the rules.
In a relationship, the rules are set by the people in the relationship.
As such, for some, having sex with other people is
NOT cheating. If the rules of their relationship allow for having sex with other people under certain conditions, and those conditions are met, then it isn't cheating.
Also, relationships can and do have rules other than "don't have sex with other people" or "only have sex with other people under these conditions." Those rules can be almost anything to which the people in the relationship have agreed, including "Don't spend more than this amount of money on something without discussing it with me first." That's a very common rule for people who have combined their finances.
Another rule that can apply to sex is "We will share sexual affection on a regular basis, as long as extraordinary circumstances don't prevent us." Thus, cheating can include one partner consistently rejecting the other partner,
cheating that partner out of something to which was agreed.
While there are common basics that should be discussed with every (potential) partner, it's difficult to discuss every possible hypothetical nuance and variation and agree to rules about each, and so being flexible about adding, removing, or modifying rules can be helpful, especially since people discover more about themselves, what they have to offer, and what the need as they age and grow. Keep in mind that most people have a tendency to have expectations and desires, and make assumptions, that go unstated, and someone can still damage trust and their relationships with the partner(s) without technically cheating. So, please, state upfront some of the basics and work from there. These might be "I am mostly monogamous and need my partner to be mostly monogamous" or "I'm ethically nonmonogamous" or "I am polyamorous" and go from there, detailing what is meant.
Strict monogamy shouldn't be expected by default; it must be something that is discussed, with an agreement being made. None of us can assume that someone we are interested in is "available" because they're not in a relationship, or that they are "unavailable" because they are in a relationship. That is something to discuss with them.