Difficult, but I'll try. Imagine Sophia who has just allowed Sam to kiss her or touch her breasts. And now she goes to a couple who are planning their wedding. That should remind Sophia of her own marriage vows. And you will feel ashamed and guilty. And she will tell herself that she has to end the thing with Sam. We assume that Cathy and Andrey will make the Parkers a covert sexual offer (swinger or ...). And Sophia notes that Liam is ready to respond. Even more - Sophia sees Liam looking at Cathy. The idea of an open marriage is back and Sophia has the feeling that Liam would accept it. Perhaps Sophia is also a little pissed off when she notices that Liam is looking after another woman while her own wife was able to save his ass on the same day.The point was that Sam could see Sophia changing but this would be a big let down.
Could you explain this please
That's why I think the event is important. It takes the pressure off Sophia of having the feeling of betraying her husband. Without that evening, Sam would be starting from scratch the next day.