Great question!
[SPOILERS AHEAD]
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[Turn back now if you don't wan't to read spoilers.]
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Whew, this is a complicated relationship. Canonically, Maxim and Nika grew up together and were one another's firsts for every facet of their sexual and romantic lives. Their bond is extremely close and no matter who or what comes in and out of their lives, they will always love each other to an extreme degree.
Their polyamory came about long before Vincent. In fact, it's something that we're interested in exploring in a Speakeasy prequel with just BL romances...
A lot of the anger and hurt about Maxim's relationship with Vincent in their route is about him feeling that he has to maintain it in order to provide for them, not because of the relationship itself. Cora's feelings are a little different since this dynamic is new to her. It leaves her confused, jealous, and second guessing herself because she isn't sure how to navigate sharing people that she's growing to love. While this is something that Nika and Maxim have already had to work through because of the expectations of the time (taking a wife). For them, it was just a reality that was neither good nor bad, because they knew that they would always have each other no matter what circumstances they found themselves in. I will say that they do enjoy having lovers outside their relationship, and have been known to even share them.
But Cora? She's a little different. She just happened to fall in love with these two men, and it likely would not have ever gone that way outside of this specific setting and with these specific men.
For the happy end, we wanted to give a sense of wholeness for the player to bring their story to a close. First with the marriage of Maxim and Cora, and then the marriage of Nika and Cora, it represents a full joining and that they've committed themselves to each other. Finding themselves happy, supported, and fulfilled, they won't seek outside relationships and instead enjoy what they've built together.
There is another ending in the game with a completely open polyamorous end...