It is a mute argument, IMHO, of what is assumed or said in marriage vows. Whether we like it or not, marriage is assumed to be a mono relationship UNLESS STATED OTHERWISE. Just as it is assumed people are straight unless stated otherwise. Is it fair? Probably not, but there it is. So to say that you never actually SAID you were mono in vows is a ridiculous argument to me.
Even if we don't agree with the assumptions made if we KNOW they are made, and do nothing to correct them at the beginning, then those assumptions are thought to be valid.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year