It's my favorite adult story trope. "You seem so happy and glowing... must be having sex" And its always about a woman, and 9 out of 10 times, its always about cheating. This might be the first time someone caught on tho. But like others have said, its her husband so... still fucked up in a different way.
I guess it does feel overused, but it is "a thing". An unloved, miserable person suddenly discover a "spark" again, that will transpire in their usual behaviour.
It's different for people in otherwise "happy" relationships, but that still want more for whatever reason. There are competent cheaters all over the place who are capable of keeping everyone happy, but it requires lots of skills and it's time-consuming. I knew a guy who would gift a very expensive and unusual perfume to his mistresses, so he could remember them and act accordingly (more so if he forgot their names, LOL).
It reminds me of some news articles about women trying to sue the company producing Viagra for "turning their husbands into cheaters". Turns out, their husbands were cheaters during all their marriage; it's just that after ED, they lost "practice" with cheating and then got caught in the act after solving their "issue"!
Now, I don't think monogamy is for every person, but it's 2025: just openly tell your partner(s) you are polyamorous and bear with the consequences.