I think originally it did boil down to Fuck Boss=save job. But when the boss keeps playing that card ...and the fact that she LIKED the Sex.. Made it a lot easer each time. That is her torment.. She loves ... or think she loves her husband. I think she does hence her inner struggle. But we see that each time is easier and easier... Now she has the excuse of open marriage.
As for the boy not being Hutch's ... that is far out there. Tony did not even know Hutch wife before she went to the office.
Not true... I met her the night before she went into the office! Joking aside though, you're right, Christian didn't even know about the existence of Vivian before she visited the office - on the one hand, that seems very odd because even if Hutch hasn't spoken about her, I feel like a boss would at least know if you're married or not; on the other hand, Christian doesn't seem to take much interest unless in his workers, so it's not that surprising after all.
The "Sawyer isn't Hutch's son" theory is too far out there for me to even entertain briefly. It's just clutching at straws because people need to find some extreme reason to explain Vivian's fast capitulation. I also don't buy the "Christian found out that Vivian cheated previously" theory (whether related to Sawyer's father or not). As well as being unnecessary complications that would go against what we knew of the Vivian/Hutch relationship before events of the story here, both theories are things that would be well hidden and I just can't see how Christian would somehow be able to uncover them in the few hours he had available to do so.
I really do think it's as simple as "fuck Christian to save Hutch's job", but the big question comes down to whether he instigated it or she did. I'm actually veering towards Vivian instigating it, her natural attraction to "bad boy" Christian (as seen in the initial office meeting) and the circumstances/his words and testing her etc., convincing her that it was Christian's idea all along.