Au idea
Remember the episode where Kagami got akumatized the second time in season 5 and Gabriel and Tomoe overheard her and Adrien talking.
What if instead of that, the idea of arranged relationships came up before they left the room where they were having their meeting talking about the Diamond Ball. Kagami and Adrien go to talk to their parents about visiting Marinette and accidentally overheard this topic, but it's not the idea of them dating that freaks them out?
Tomoe mentioned to Gabriel before they got there that while a relationship between Adrien and Kagami could be beneficial to them, she had another suitor in mind for Kagami and was trying to figure out which would be more beneficial/have better long-term consequences.
When Gabriel gets upset and asks who it is he gets even more upset because it was the other suitor he was thinking of for Adrien.
This suitor was no other than Marinette Dupain Cheng.
So, Kagami and Adrien walk into a whole argument about who Marinette would be a better partner for, who gets dibs on her.
Tomoe has an idea and is like, well, "polyamory runs in your family....and Paris has become more accepting...a throuple could be seen as very progressive and we'd benefit on all sides".
Gabriel wants to know what in the ever loving fuck she's talking about, and how she knows what a damn throuple is, and she mentions him, Emilie, and Nathalie.
Gabriel gets all confused and defensive because they weren't together (he's extra defensive about the implication that he could have feelings for anyone but Emilie), and Tomoe makes a comment about how she doesn't need to see to see that steaming pile of bullshitery. As well as the fact that there is no heterosexual explanation for the way Emilie and Nathalie would act around each other.
Obviously, Nathalie was a major part of their marriage.
Gabriel's having a whole crisis, Tomoe can't believe that he's not only blinded than her but is also even more idiotic than she thought, Adrien just wants to know what's going on, and Kagami is filming so she can show their friends to prove that their parents' are, in fact, as wild as they've both told them.
TLDR: Gabriel and Tomoe both want their children to date Marinette. Tomoe decides that a throuple could work. Gabriel wants to know how she knows what that is. Sangreste is obvious to everyone that isn't Gabriel. Adrien and Kagami are just watching this fight go down.