Lets break it down the psychology behind the events and why she choose the actions she did.
With story as is shes implied to be under immense stress (the move into a new home, the blackmailing over her son, worried about her husband being around much younger and far more superior female specimens while at work, the kids always at one another throats the list goes on and on).
1. These things add up (stress just doesn't just happen over night)
2. While under stress everyone looks for outlets known as "escapism" (sexual, mental, physical or a combination of)
3. Stress can corrupt, make good people do horrendous or out of character actions (You give a choice between someones self or strangers survival during a stressful event think "Lost", where few reasonable selfs win out and even then there were doubts.)
4. Affection, if someone is "feeling" (mentally as in an older women whom no longer sees herself as a sex object) a longing for "it" (that sexiness that she feels has lost "it") and someone seeds and waters that longing for being looked as sexy once again (may take some coaxing and time but damn skippy they going to pounce on it.)
Just go to a bar on any giving Thursday or Friday night just before closing you see the examples of this in living color.
5.When you work a tightly confined socially rigid and doctrinated job like a teacher or cop, somethings bound to break (usually that something is those social confines of morality if not mentality first, like a teacher fucking a student or if they have a weak mental state will default to anger "going postal" as its said in popular culture).
6. Late onset sexual confusion can be brought on by stressful or past irreconcilable events that has happened in life or that have recently been triggered by consent daily stressors.
I haven’t seen this discussed here before, sorry if it has – “The Twenty-Year Itch”, which is a chronological companion to “The Seven Year Itch” and “The Honeymoon is Over”. I don’t even know if The Twenty-Year Itch is an accepted thing, or just someone’s theory. The little bit I’ve read is that it’s a time where one or both spouses have grown bored or tired of their marriage, and are rethinking the relationship or letting things slide, and wondering if it they made the right marital choice all those years ago.
I have no idea if L&P is aware of this, and for me it doesn’t matter. When one is looking for reasons why, seemingly all of a sudden, a “happily married” woman with a near-ideal lifestyle would stray, I heap this theory into my Plausibility Bank to make the story feel less far-fetched. Especially if both Sophia and Liam are suffering from this, at least for the moment the romance is gone, and when faced with marital challenges both are more prone to think “Why not? What have I got to lose?”
Maybe this phase can help explain why Liam isn’t as interested in Sophia; why he risks gambling away all his money, why he would consider trading away Sophia for a chance to have sex with Cathy.
Sophia isn’t yet to the point of throwing away her marriage. She still worries about getting caught with other men/boys and losing her husband. But the rewards are becoming greater than the risks. She is much more easily swayed in erotic moments; and more likely to put herself in erotic situations.
As I said, just another theory to make this game more plausible in my own mind.