I just posted on your intro thread - didn't realise that this was here.
How many times have you heard the old one about "marriage broken - let's have kids - that'll fix it!" and you just know that it's not really going to fix anything.
A similar saying goes for poly "marriage in trouble - add more people - that'll fix it!" - it doesn't apply either.
When trust isn't there... when someone is doing it because they can't "get what they need at home" those are not bases for good, stable polyamorous relationships, but recipes fo on-going drama.
Most people that come to poly this way (and I am one of them) really need to realise that the cheater has done something to damage the original relationship and that this has to be repaired before anything polyamorous is considered (and no, I didn't do that either, and paid the price for it).
The big thing is that in any loving relationship, trusting each other is vital. if that isn't there, then poly is going to be a very hard, or impossible slog.
"Listen, or your tongue will make you deaf." - Native American Proverb