It's an inconvenient truth that cheating doesn't always stop once one becomes poly, and is something we would rather not discuss. After all, a main differentiation point for poly is that WE are honest in our dealings within our relationships, so cheating behavior is pretty thoroughly scorned, disavowed, and abhored within our community. Still, it's certainly not unheard of for people who would prefer to think of themselves as ethically non-monogamous to backslide in those ethical ideals, and stray towards cheating behaviors.
It's confused me. We're poly! Our current relationships are all set up that way. We have all this freedom! Why fuck it up? Why would one ever choose to skip the consent and information sharing section in favor of deceit and deliberate damaging of trust built in their life? Here are a few ideas I've observed being bandied about:
I'm not willing to do the work to integrate a new person into my existing life. Either there's a lack of communication skills with one or more involved, or a difference in poly styles that makes the new person incompatible with what currently exists.
I get off on having a Dirty Little Secret. It feels naughty, taboo, and there is still a charge there, the thrill of having a secret, getting away with something.
It's just faster to get what I want, right now, without the "burden" of consent. I want what I want, and I want it now!!! Why should I take the time to send that text message to my partners, or delay the gratification I seek in the now? I'm a free person, and I can do what I want, when I want, with whom I want, and I'm not sure this opportunity will be there again, if I wait.
I still feel like it's "wrong/selfish" to desire another person. There are often mono societal tapes playing in the background, usually with everyone involved, that lead towards guilt. What they don't know, won't hurt them!
I'm doing this for the protection of my existing partner(s). They're sick/pregnant/unable to consent in some fashion. It's finals week. The timing is just awful. Seriously. It'd be selfish for me to bring this up right now. I'll double back later.
I've made an agreement that I didn't really desire personally, and don't know how to/if it can be successfully renegotiated. This one is tricky. You said yes, and often those sorts of agreements are made to protect your existing partner from something that is hard for them to do, and something you understand the reasons behind. To change those agreements will take buy in from your existing partner(s), and you may not feel confident in their willingness to do that, or your own worthiness of their effort in this regard.
I fear that a current partner might look at my new dating choice and feel like I'm missing something major that would preclude that being a good idea to pursue; the Jiminy Cricket Effect. Most of us are aware that our own judgment can be skewed in the early stages of a relationship, and we may very well be ignoring major, flashing, neon signs that read, "Path to destruction!". It's harder to ignore those signs when an outside, loved, and trusted source, is pointing them out to you, and also mentioning that they are likely to harmed in this as well. Better to just sidestep that inconvenience...
There's a semi-comprehensive list of reasons for cheating in poly! Wasn't that fun? That whole section is just a recitation of justifications for poor behavior. It's bullshit rationalizing to get your own way, without considering larger ramifications. If your brain, or body part of your choice, is headed down any of these pathways, back up the boat!
I've never regretted putting off sex, or not moving faster in a new relationship to attend to the needs of my current partners and relationships, even when I'm crawling out of my skin with NRE. It's a big old lie based in scarcity thinking that everything worth having must be had NOW, or it will evaporate into the ether. Take the time! Do the work! Get some new skills, if you're in over your head! Find a mentor! If you're unable to renegotiate, move out of the current relationship before you embark on the next! Delayed gratification will not kill you. Hold the line. Don't cheat.