How many women had affairs? Were they all "middle-aged"? That's a very general sweeping statement. Over a long & chequered career I encountered a few people having affairs, certainly. There was no real common denominator & those involved were as varied as the rest of us, as were the motivations concerned. Elements like the menopause, 7-year itch etc., are just trotted out as weak excuses. Straightforward sexual temptation is usually the start point for whatever gender or age the participants are, or in other words, it's about pleasure seeking. I don't believe that particular types, or age groups are more prone to these affairs than others. "Sick of the marriage" is the soundest reason you give.
I also worked with women who looked & dressed as good as Sophia in the latest spoiler (& I mean a lot). They weren't desparate to prove anything, as they didn't really have to & whether they contemplated a fling with a stranger, I've no idea. They never said. If they indeed took part in some of the actions you list, all I can say is that it was working.
L&P ideally should provide some solid reason(s) for Sophia to have an affair or two. At present it's all about her being self-centred & becoming bored with a husband she feels is too absent &/or under-performing. Maybe this will have to be enough but IMO, she wouldn't launch into an affair with her son's friend in the expectation that a 17 year old virgin was going to satisfy her sexually for example! Also, how could Sam support her in the style to which she's become accustomed? Reasons become even harder to find for why she might embark on incestuous, potentially life-destroying liaisons with blood relatives, Again, it has to be about pleasure in the moment & to hell with the consequences.
I will say, however, that I totally agree with your choice of shoes to go with that suit.