Why is it likely to be Christian? It's more likely that, if it's sexual/romantic, it will involve Hutch. If he's getting attention (whether he encourages it or not) from someone, say Addison, and Vivian sees or is otherwise aware of this, she is likely to be jealous. And it doesn't matter that she has been doing stuff (with Christian), as jealousy isn't a rational emotion.
I'm actually glad you pulled me on this as I wasn't happy with how I explained it so currently as it stands with what's been released so the main sort of focus has been Vivian and Christian , now I'm almost certain that the Dev said the next update will be more around hutch or something along those lines so with out seeing it id be going off what I have seen so far
So Vivian and Christian are essentially like new shiny toys for each other , it's like any new relationship in real life it's hot , passionate, wild ,thrilling etc
Where as for Vivian and hutch yeah they have likely got themselves in a rut after all they have been married 22 years I would say it's only natural that something like that could happen
Now if we think to the living room scene where hutch opens the marriage, there is a part where Vivian suggests that if hutch needs to do to her what she has done to him that she would understand and wouldn't hold it against him or something loosely along those lines , now I can't remember if hutch didn't actually acknowledge or answer that question or just skipped past it , but what I can say is it didn't suggest that something like that was on his mind , now this isn't to say that won't change who knows
So based on that it would suggest/ lay the ground work that Vivian has no cause for concern that hutch will act outside of there marriage while it is open and again that could change but like I said this is purely off the information available at the moment
So with that even if we just had hutch and Christian side by side realistically who out of the two is more likely to play around
And to add to it one thing I've always felt about Christian is I don't think he actually cares about Vivian the vibes I get from his character are that he is incapable of caring about or loving someone that isn't himself essentially it's Christians world we all just happen to live in it sort of thing
Hopefully that helps with what you where looking for in regards to it but let me know your thoughts around I'd be more than happy to discuss further