Hello people of poly advice

Full disclaimer, I am not poly and don’t pretend to understand how it works, which is what has brought me seeking advice/education on a situation with a friend that seems problematic to me so I thought I’ll bring it here for education.

Two friends of mine (both late 30s/early 40s and both female) recently got married, but 2 days before the wedding one of the brides, G, was bursting into tears. I asked here why, and I was informed that her partner, D, had brought a new partner, A, into their poly relationship when they had agreed that they wouldn’t look additional partners until after the wedding as it was a day that was supposed to be all about them.

According to D it’s alright because she and A are not ‘official’ yet I.E. they are not sleeping together but to me they are still acting like a couple; hand holding, snuggling and pecks on the cheek etc, and it was quite obvious that is was upsetting G.

Now as an exclusive person I don’t have the experience/education to judge a poly relationship but from my outsider perspective this doesn’t seem right. Just off the cuff, if something is upsetting your partner so much they are bursting into tears it should at least be discussed between them and if the agreement was to not look for/bring in new partners till after a certain time then it’s reminding me of the Boxing Analogy: if both people agree to it, it’s a sport. If only one person dose, it’s assault. Or in this case if both agree to it, it’s polyamory, if only one does it’s cheating.

Am I simply an ignorant monogamous person or is this situation fucked up?

Edit for more info: G is gay while D is bi making A specifically D’s partner.