Originally Posted by Chimera
There's often a lack of clarity around boundaries -- how one person understands a conversation about the relationship might be distinct from the other partner. Or one person has personal rules/boundaries that they don't really communicate to the other person, but assume that they're in agreement. There are times when the "cheating" is really a misunderstanding.
There is sometimes assumed monogamy also. Sometimes people just don't talk and expect that each other is doing what THEY think is appropriate. I have noticed that it is sometimes very convenient to NOT talk about something in order to get ones way and use that as an excuse to cheat.
Some people tell their partner they are poly and that is it. They don't expand on that or let the partner know what is going on for them and the partner is left in the dark about what is going on. Sometimes that leads to them believing that their partner is poly but has decided to be monogamous with them. Worse than that, that their partner has decided to not talk to them about it. Its not lying, but its just as bad in my book.