It seems to me like the most reasonable defense against an adultery charge for a couple entering a legal marriage in the US is to have a prenup that clearly delimits how to parsel joint resources back out in the event of a divorce. IMO, marrying couples of any lovestyle with any real funds to speak of need to get a prenup, just so that both sides have to have a conversation about how they both feel about the money they have, and to keep their expectations together.
None of that protects children in a divorce where poly is involved in the US (and I suspect in many other places). According to the Wiki prenup page
Prenuptial agreements in all U.S. states are not allowed to regulate issues relating to the children of the marriage, in particular, custody and access issues. The reason behind this is that matters involving children must be decided in the children's best interests.
Of course, nothing but a good lawyer and a sensible judge seems to protect poly parents when their fitness to parent becomes a legal question.
At the end of the day, I have to agree with SchrodingersCat:
Don't marry the sort of person who's likely to stab you in the back if things don't work out.
Go in with your eyes open, and make all the necessary arrangements (including your wills), and then surround yourself with people who will respect the agreements you've made, legal or no.
@jackrabbit, I'm getting the sense you're curious about this topic academically... Am I off the mark? (Sorry for prying, but it's hard to give advice at a theoretical level sometimes.)