I've searched but can't find any similar bugs, & while I run about 40 mods I'm fairly sure this is an Ideology bug.
Situation is that I have several ideologies co-existing:
Archotech ideology 1 allows 1 spouse for male & female
Archotech ideology 2 allows 1 spouse for female but 2 for male
Pawn 1 is male of archotech ideology 2, & has 2 girlfriends, both of ideology 1.
He proposes to, & is accepted by both females: 1st marriage goes ahead as normal, 2nd marriage (a few days later) is ignored by entire colony except for the 2 participants.
In addition to the non-attended marriage issue, I think wife #2 is breaking her own Ideology precepts which only allow 1 spouse per male/female. They usually follow these rules religeously, as if one ideology allows free love, & the other retricts it then 'new lovers' won't sleep together.
I can add more detail/save/mod list etc later.
Thanks for the report. Since this does come from a modded playthrough, I am going to put this on my document as something to look out for while testing, I appreciate you giving a clear indication on the exact way this happened though for me to do some testing.