Addressed generally to Jodi, as an afterthought:
It really is hard to come to grips with moral guidelines that fall significantly outside your own. As an anecdote that you might find even more incomprehensible…
My wife and I, in addition to being polyamorous, are also to a certain extent swingers (that being the sexual-attraction-only variant, of course). While I’m not surprised to find that polyamorous couples are generally respectful of other couple’s boundaries, what has consistently pleasantly surprised me is how often a group of swingers, who are ostensibly only in it for the sex, will nonetheless ostracize any person in the group who’s been found to be dishonest with their spouse or cheating without permission.
The more I think about it, the more I think it’s probably that polyamory requires a good deal of honesty and clear boundaries in one’s primary relationship for it to function properly, and that seems to naturally translate for a good many people I know into respect for honesty and boundaries belonging to other people. As noted in the examples upthread, the only “poly” couples I know in which one or both participants is a homewrecker are also ones in which one participant is totally uncomfortable with the whole thing to begin with, which isn’t so much polygamy as it is emotionally blackmailing your partner into letting you cheat, IMHO.