Same way you cheat in a monogamous situation: break the ground rules of the relationship.
Some people have rules about circumstances under which people can get involved with new people – ‘only with my consent’, ‘only with prior notification’, ‘only in a less committed relationship’, ‘not without current STD testing covering these conditions’, or even ‘not at all’, as some poly families are polyfi (closed to new relationships); starting a new relationship in violation of those rules is cheating.
Some people have rules about the sorts of things one can do in other relationships – ‘no sex’, ‘no coitus’, ‘no BDSM’, ‘BDSM only’, ‘you can’t have a date at our favorite restaurant’, ‘you can’t have sex unless I’m involved’, ‘no sex in our bed’, ‘no sex when I’m in the house’, ‘no sex without using a barrier’. A friend of mine had an agreement with her husband that he was the only person who got to trim her hair.
Some people have temporary conditional states in which they’ve got additional rules – ‘no sex with your other partners while we’re trying to conceive a child’, ‘limit time with other people while we’re working on this relationship issue’, ‘no going out on dates while my mother is visiting’.
My personal rules (that I can remember; I operate rule-light and don’t spend much vigilance on these things) are:
- barrier protection for sex outside the family (who counts as ‘family’ is negotiable)
- current STD tests when new relationships begin
- people are informed of significant relationship developments as they happen
- I prefer not to be in the company of people who must be prevented from knowledge about relationships; I absolutely will not accept their hospitality if I am expected to lie to them
- I may, at times, make agreements restricting my own behaviour or ask for same from partners; for the most part, these are temporary, but some have lasted the duration of the relationship (case of a partner saying, ‘I can’t deal with it if you do X’ and me agreeing not to do X)
I have dealt with a variety of other restrictions in the past, including things like ‘no sex in our bed’ (which is a rule I find silly, but will respect); there exist in theory rules that I would not have a problem abiding by (like ‘no sex outside the family’) which are not in play in my life.